Well Logging

Sedimentary rock is a type of rock that is formed by sedimentation of material at the Earth's surface and within bodies of water. Sedimentation is the collective name for processes that cause mineral and/or organic particles (detritus) to settle and accumulate or minerals to precipitate from a solution. Particles that form a sedimentary rock by accumulating are called sediment. Before being deposited, sediment was formed by weathering and erosion in a source area, and then transported to the place of deposition by water, wind, mass movement or glaciers.

Azlin Samsudin, Tarmiji Masron, Mokhtar Saidin, Jeffrey Abdullah and Saiful Shahidan.(2010). LEMBAH BUJANG, SEBUAH KERAJAAN LAMA YANG BERPUSAT:
DARI PERSPEKTIF SISTEM MAKLUMAT GEOGRAFI (GIS).IN: Seminar Prosiding: Penanda Arasan Penyelidikan Arkeologi di UKM (Editor: Asyaari Muhamad, Nik Hassan Shuhaimi Nik Abd. Rahman and Zuliskandar Ramli).Institut Alam Dan Tamadun Melayu (ATMA), Bangi.Pages: 305 - 325

Abstract:

Lembah Bujang telah dikaji sejak sedekad yang la/u dan dikatakan sebagai kerajaan Kedah Tua dari abad ke 1 masihi hingga 14 masihi. Ia juga dilaporkan sebagai salah satu kerajaan lama yang penting dalam abad tersebut sebagai salah satu persinggahan dalam perdagangan di rantau Asia Tenggara. Ma/ah dikatakan juga, penempatan di Lembah Bujang adalah merupakan satu kerajaan atau negeri yang mempunyai penyusunan masyarakat yang tersusun dan jaringan pertukaran barangan tersendiri dengan menggunakan sungai-sungai sebagai penghubung antara kawasan. Fungsi setiap tapak juga telah ditafsirkan berdasarkan kepada kajian o/eh Jane Allen pada tahun 1988. Kesemua tersebut adalah hasil daripada kajian arkeologi melalui  tinjauan dan ekskavasi yang dilakukan oleh pengkaji-pengkaji terdahulu. Bagaimanapun, kajian terhadap taburan tapak-tapak arkeo/ogi di Lembah Bujang dengan menggunakan pendekatan Sistem Maklumat Geografi (GIS) tidak pernah dilakukan /agi. 0/eh itu, ana/isis GIS telah dilakukan ke atas taburan tapak-tapak arkeologi yang sedia ada untuk menentukan polo penempatan dan pusat penempatan tersebut dan sungai utama yang menjadi nodi dalam kerajaan lama. Ana/isis yang dijalankan menunjukkan taburan tapak-tapak adalah tertumpu pada dua lembangan iaitu kawasan tadahan sungai Merbok dan Sungai Muda. Kedua-dua sungai ini memainkan peranan dalam kerajaan Kedah Tua. Ana/isis terhadap polo taburan tapak-tapak arkeologi pula berdasarkan kepada 3 atribut iaitu ketinggian, kecerunan dan fungsi setiap tapak menunjukkan bahawa wujud polo penempatan dan pusat kerajaan Kedah Tua. Polo penempatan Lembah Bujang ada/ah berkisar kepada Sungai Merbok sebagai sungai utamanya berbanding dengan sungai Muda. Ma/ah ana/isis tersebut juga menunjukkan bahawa kawasan Pengkalan Bujang dan sepanjang sungai Pengkalan Bujang kemungkinan merupakan pusat kerajaan Kedah Tua. Sekitarnya merupakan kawasan aktiviti ekonominya. Kewujudan pusat kerajaan, menunjukkan wujudnya kerajaan lama dan penempatan di Lembah Bujang merupakan sebuah penempatan satu negeri yang mempunyai kerajaan berpusat.

Ditulis pada Maret 30, 2012 BAB 1 PENDAHULUAN 1.1 Latar Belakang Masalah Saat ini teknologi di dalam eksplorasi dan eksploitasi minyak dan gas bumi telah berkembang dengan pesat. Hal tersebut sangat diperlukan mengingat harga minyak dan gas bumi yang semakin meningkat sehingga perlu dilakukan eksplorasi terhadap sumur minyak baru maupun peningkatan produksi terhadap sumur minyak yang telah ada sebelumnya. Sebelum dilakukan pengeboran kita harus melakukan evaluasi formasi untuk mengetahui karakteristik formasi batuan yang akan di bor. Berbagai macam metode digunakan untuk mengetahui karakteristik formasi baik melalui analisis batu inti, analisis cutting, maupun analisis data well logging.

This paper aims to review some of the widely used conventional and other contemporary Inflow & Productivity models that exist for Horizontal wells, and comment on their limitations by drawing on the experiences of other author's critiques on these IPR models.

The dielectric constant of a material affects the way in which electromagnetic wave passes through it. Since the dielectric constants of oil and water are different, the behavior of electromagnetic waves in reservoir rocks is of interest. Based on dielectric measurements, two classes of tools currently exist viz.: very high, and low frequency tool. They all use small antenna arrays to send electromagnetic waves through the formation. Traditionally, the measurement of the conductivity or resistivity of a formation has been one of the main surveys performed in a borehole, primarily to determine water saturation. However, a second electrical characteristic of the formation can be determined – the dielectric permittivity. Dielectric devices can be used to determine formation saturation from data dependent on the dielectric permittivity. The basic measurements are of propagation time and attenuation of an electromagnetic wave as it passes through a specific interval of formation. As the propagation time in water is higher than in hydrocarbons or minerals, the measurement is primarily affected by the water-filled pore space of rocks. This is in contrast to the nuclear-porosity tools affected by the total porosity. In addition, for a wide range of salinities, the propagation time in water is practically constant and so saturation estimations can be made without prior knowledge of the resistivity of the formation water. When other open-hole log data are available, it is possible to distinguish between oil, gas, and water with unknown changing water salinity. There are errors associated with conventional porosity tools. The dielectric dispersion logging tool is a relatively new tool that overcomes major problems of pre-existing logs. The tool uses electromagnetic wave to detect water-filled porosity.

The identification of fluid contacts (gas–water contact—GWC, oil–water contact—OWC and gas–oil contact—GOC) is essential for field reserve estimates and field development and, also, for detailed formation evaluation. For the accurate calculation of some petrophysical parameters, such as porosity, the reservoir interval has to be zoned by fluid type, to account for differences in fluid saturations and fluid properties (e.g., hydrogen index, density, sonic transit time) in the various intervals: gas cap, oil column and aquifer zone. The fluid contacts may vary over a reservoir either because of faults, semipermeable barriers, rock quality variations / reservoir heterogeneity, hydrocarbon-filling history or a hydrodynamic activity. Horizontal contacts are typically taken into consideration, although irregular or tilted contacts occur in some reservoirs. The methods used for determining the fluid contacts include fluid sampling, water and hydrocarbons saturation estimation from geophysical well logs, analyses of conventional or sidewall cores, and formation pressure measurements. The pressure profiles obtained with various formation testing tools over reservoir intervals are, frequently, the primary source of data for defining the fluid contacts. When good quality pressure data can be collected, the fluid contacts can be determined by identifying the depths at which the pressure gradients (pressure versus depth trends) change. This study addresses some issues related to the identification of GWC for two gas fields of Early Pliocene age (Dacian stage), belonging to the biogenic hydrocarbon system of western Black Sea basin-Romanian continental shelf. We show that the identification of these contacts based exclusively on pressure gradients analysis is uncertain or may be inaccurate. The pressure gradients approach should be checked against the results of the conventional interpretation of geophysical well logs (e.g. changes in the computed fluid saturations as a function of depth) and, if available, the results of nuclear magnetic resonance (NMR) log investigations, which are able to indicate the intervals with clay-bound water, capillary-bound water and movable fluids.

Overpressure merupakan salah satu fenomena geologi berkaitan dengan nilai tekanan yang secara signifikan berada di atas gradien hidrostatik normal. Kehadiran overpressure dalam sebuah cekungan sedimentasi dapat diakibatkan pembebanan sedimen (loading) atau akibat unloading. Kehadiran overpressure dalam Cekungan Kutai Bawah dan Cekungan Sumatera Utara dapat menyebabkan gagalnya pengeboran dan membahayakan proses eksploitasi minyak bumi dan gas pada tempat tersebut. Oleh karena itu pemahaman mengenai overpressure ini sangat penting bagi seorang engineer dalam melakukan eksplorasi dan eskploitasi di daerah cekungan sedimentasi. Cekungan Kutai Bawah terletak pada bagian timur Pulau kalimantan sedangkan Cekungan Sumatera Utara terletak pada bagian timur laut Pulau Sumatera. Untuk menganalisis Overpressure dapat dilakukan dengan plot silang tekanan-kedalaman, plot silang transit time-density¸ plot silang resistivity-density, analisis wireline log density, sonic, resistivity, hingga reflektansi vitrinit. Analisis dilakukan pada tiga sumur berbeda pada Lapangan Peciko, Cekungan Kutai Bawah dan dua sumur pada Lapangan Langkat, Sumatra Utara. Pada Lapangan Peciko, Cekungan Kutai Bawah terdapat overpressure dengan mekanisme unloading yang disebabkan oleh transformasi smektit menjadi illit dan gas generation. Pada Lapangan Langkat, Cekungan Sumatera Utara terdapat dua mekanisme overpressure, yaitu mekanisme unloading yang disebabkan oleh transformasi smektit menjadi illit dan hydrocarbon generation, serta mekanisme loading yang disebabkan oleh batuan yang gagal kompak.

Kata kunci: overpressure, Cekungan Kutai Bawah, Cekungan Sumatera Utara, unloading, loading.

Carbonate reservoirs are important exploration targets from the perspective of hydrocarbon and, also, groundwater reserves. In this respect, the most important aquifer system in Romania is located in South Dobrogea region, comprising two distinct aquifer complexes: a deeper one, mostly under pressure, hosted in fractured/fissured and karstified limestones and dolomites of Late Jurassic–Early Cretaceous age and a shallower one, mostly unconfined, hosted in Sarmatian (late Middle Miocene) lumachellic and oolitic limestones. The deeper aquifer is regional, confined on more than 60% of its extension, has a SW–NE general flow direction and discharges in Siutghiol Lake (Constanţa city area) and through submarine springs on the Black Sea continental shelf. The water flow occurs through fractures/fissures and dissolution voids within the carbonate rocks and, also, along fault planes.
Although a large number of wells were drilled in South Dobrogea for groundwater exploitation from the Late Jurassic–Early Cretaceous aquifer and, also, for groundwater quality monitoring, there are few published data and studies regarding the geophysical response and petrophysical features of these formations, in relation with their hydrogeological characteristics. The more recent drilling of three hydrogeological research boreholes in the proximity of Cernavodă Nuclear Power Plant (NPP) offered an opportunity to gain valuable information concerning the geological succession in this northern part of South Dobrogea, with special regard to the carbonate formations hosting the aquifer. The boreholes were drilled to a depth of 700 m, for the identification and exploitation of aquifer horizons, in order to provide a fresh water supply for the NPP units. Geological analyses on drill cuttings and cores and geophysical well logging were carried out, for a comprehensive characterization of the intercepted formations from a lithologic and stratigraphic viewpoint, the identification of fractures/fissures and caverns as potential water-producing zones and the evaluation of formation porosity. The geophysical logs interpretation showed that the most probable water-producing zones are located in the so-called Dolomitic Complex (Kimmeridgian–Tithonian) on the 620–700 m (particularly 650–700 m) depth interval, in carbonates with average porosities of 12–16% and locally reaching more than 40%.

"Abstract: Shaly sandstones are complex formations. They consist of rock particles and shale component.
Substantial property of shale component is the ability for selective adsorption of ion from the aqueous solutions
that contact with shaly sandstones. Experimentally was obtained that electrical conductivity of the shaly
sandstone increases by nonlinear way with increases in conductivity of the brine. In 1966 Waxman and Smits
reported a new equation for electrical conductivity of water saturated shaly sandstones based on the Waxman
and Smits Model (WSM). It includes a product of Cation Exchange Capacity (Qv), the maximal equivalent ionic
conductivity of the sodium exchange ions ( 
Na  ) presented by the parameter B and the Formation Factor F*. It is
supported by laboratory data on core samples. Analyze of WSM points to the limits of this equation. It is shown
that B has a specific value for every group of core samples. Data scattering is valuable in low Qv values. It is
outlined that Qv is obtained by destructive core analyzes. It is offered a new equation that explains the non
linear part of electrical conductivity in the WSM. An idea of “ideal shale” and the upper limit of 5,43gequiv/l of
Qv is applied. The Dual Water Model (DWM) and WSM are compared. Some analogy between them is
discussed. It is shown that the electrical conductivity of adsorbed water on clay minerals can varies within
25S/m to 10S/m at 250C. It is due as to exchange cations as to adsorbed anions on clay particles. An equation for
CAW is derived. It is shown that F* depends on Qv as well. It is underline the valuable role of WSM in the
evolution of ideas for electrical conductivity of shaly sandstones.
Key words: Shaly sandstone; Electrical conductivity of shaly sandstones; Waxman and Smits equation;
Dual Water Model."

The study of groundwater potential in Wadi El-Assiuoti is of a prime importance for both current and future development. In this paper the groundwater occurrences are investigated using integrated; 1) geophysical well logging (Self potential, Resistivity and Gamma Ray), 2) subsurface lithologic data and 3) hydraulic data (trans-missivity, transmissibility, hydraulic conductivity and storativity) in addition to reviewing all studies on Wadi El-Assiuoti. Different petrophysical parameters (formation water resistivity, formation factor, porosity, perme-ability, salinity and shale volume) are computed. These parameters are very necessary to identify the aquifers characteristics in the surveyed area. The integration between geophysical and hydrogeological data shows the possibility of two water bearing formation (Quaternary and Plio-Pleistocene). Generally, porosity and perme-ability values of these aquifers decrease toward the west. The aquifers exist under unconfined to semi confined conditions. The wide range values of both well efficiency and hydraulic conductivity indicate heterogeneity in the lithology and hydraulic characteristics of the aquifer materials. The computed transmissibility values indicate that the water-bearing sediments in the area under investigation are generally of high productivity. In some localities due to the heterogeneity of sediment, the water-bearing sediments are of moderate productivity. Recently , due to the increase of drilling activities at the middle part of the studied area some problems concerning the groundwater potential and high salinity in the area are happening.

This paper describes how distributed temperature sensing (DTS) based on Raman Scattering is being used as an in-situ logging technique in oil & gas wells. Traditional methods of gathering production data to characterize oil & gas well performance have relied on the introduction of electric logging tools into the well. This can be an expensive process in highly deviated or horizontal wells and usually results in the well being shut-in with the loss or deferment of hydrocarbon production. More recently permanently placed pressure sensors based on CMOS technology have been used, but these systems do not easily deliver distributed measurements and reliability has been found to be poor.

New high-resolution seismic and swath bathymetry data reveal the detailed structure of nine mud volcanoes and two large fields of pockmarks at 400–1100 m water depth in the Ceuta Drift (Western Alborán Sea). These seepage features are related to episodes of re-activation of focused fluid flow throughout Pliocene-Quaternary drift deposits. We describe the key structural elements and seismic signatures of nine mud volcanoes systems that comprise: (i) cones and bicones as constructional edifices and low-dip muflows interfingering with adjacent drift sediments (ii) feeder pipes and unstratified cone-shaped columns as feeder complexes underlying the extrusive edifices. All these elements reveal the evidence of extrusive activity over time and thus are used to address an approach to time the reactivation episodes. According to their structural elements, these mud volcano systems are grouped into four major types: I: Complex edifices; II: Stacked single edifices; III: Single edifices fed by buried bicones; IV: Single edifices. Correlation between mud extrusion episodes and main drift discontinuities allow us to constrain the age of reactivation episodes of mud extrusion along Pliocene-Pleistocene times. Complex mud volcanoes composed of twin to multiple seabed cones seem to be the longest-lived systems active since the mid-Pliocene times with, at least, six episodes of reactivation. Most of mud volcanoes have been reactivated in the mid-Pleistocene (~ 0.9 Ma, MPR discontinuity) and all mud volcanoes in the MIS12 (0.4 Ma). Even though the onset of focused fluid flow might have been triggered by mid-Pliocene tectonics and diapirism, we suggest that some of these generalized phases of mud extrusion might also be related to erosion associated to major sea-level falls.

In addition to vertical focused fluid flow, we propose that leakages from the mud volcano feeder complexes into high-permeability contourite sediments may cause lateral fluid migration and form gas accumulations. Lateral fluid flow in the near surface sediments could be responsible of forming subsurface reservoirs and consequently, random pockmarks by fluidification and/or liquefaction of the non-consolidated shallow sediments. Stacked buried palaeo-pockmarks indicate that this mechanism has been persistent along recent times, at least after the mid-Pleistocene.

Maintaining a stable borehole and optimizing drilling are still considered to be vital practice for the success of any hydrocarbon field development and planning. The present study deliberates a case study on the estimation of pore pressure and fracture gradient for the recently decommissioned Volve oil field at the North Sea. High resolution geophysical logs drilled through the reservoir formation of the studied field have been used to estimate the overburden, pore pressure, and fracture pressure. The well-known Eaton's method and Matthews-Kelly's tools were used for the estimation of pore pressure and fracture gradient, respectively. Estimated outputs were calibrated and validated with the available direct downhole measurements (formation pressure measurements, LOT/FIT). Further, shear failure gradient has been calculated using Mohr-Coulomb rock failure criterion to understand the wellbore stability issues in the studied field. Largely, the pore pressure in the reservoir formation is hydrostatic in nature, except the lower Cretaceous to upper Jurassic shales, which were found to be associated with mild overpressure regimes. This study is an attempt to assess the in-situ stress system of the Volve field if CO 2 is injected for geological storage in near future.

Kajian arkeologi di Lembah Mansuli, Lahad Datu, Sabah Ielah menemukan
dua tapak Paleolitik, iaitu Gua Samang Buat dan tapak terbuka Mansuli. Isu utama
di Gua Samang Buat ialah berapakah kedalaman sedimen di dalamgua ini selepas
ekskavasi empat meter masih belum menemui lantai gua. Manakala isu utama di
tapak terbuka Mansuli ialah interpretasi paleoalamnya yang dikatakan bersekitaran
tasik kuno. Maka, kaedah geoftzik telah digunakan untuk cuba menjawab isu ini.
Penggunaan kaedah geofizik di Lembah Mansuli telah berjaya membantu
mengenalpasti ketebalan sedimen di Gua Samang Buat dan membantu membina
paleoalam untuk Lembah Mammli. Ketebalan sedimen gua di Gua Samang Buat
yang boleh di' ekskavasi ialah 12 m di Gua 1 dan 5 m di Gua 2. Data ini
mencadangkan secara relatifberkemungkinan kita boleh menemui lapisan berusia
138,000 tahun dahulu di Gua Samang Buat berdq~rkan usia 46,000 untuk
kedalaman 4 m. Kajian gwifizik di Lembah Mansuli pula mendedahkan wujud
tasik kuno dan sa/iran sungai kuno di Lembah Mansuli. Malah daripada survei
tersebut, ianya juga mendedahkan kemungkinan wujud dua teres tasik kuno. Modelanomali graviti dan magnet juga turut menghasilkan hentuk tasik.

The objective of this study is to provide information on source organic matter input, depositional conditions and the correlation between crude oils recovered from Sunah oilfield and Upper Jurassic Madbi Formation. A suite of twenty-six crude oils from the Lower Cretaceous reservoirs (Qishn clastic) of the Masila Region (Eastern Yemen) were analysed and geochemically compared with extracts from source rock of the Upper Jurassic (Madbi Formation). The investigated biomarkers indicated that the Sunah oils were derived from mixed marine and terrigenous organic matter and deposited under suboxic conditions. This has been achieved from normal alkane and acyclic isoprenoids distributions, terpane and sterane biomarkers. These oils were also generated from source rock with a wide range of thermal maturity and ranging from early-mature to peak oil window. Based on molecular indicators of organic source input and depositional environment diagnostic biomarkers, one petroleum system operates in the Masila Region; this derived from Upper Jurassic Madbi organic-rich shales as source rock. Therefore, the hydrocarbon exploration processes should be focused on the known location of the Upper Jurassic Madbi strata for predicting the source kitchen.

The dielectric constant of a material affects the way in which electromagnetic wave passes through it. Since the dielectric constants of oil and water are different, the behaviour of electromagnetic waves in reservoir rocks is of interest. Based on dielectric measurements, two classes of tools currently exist viz.: very high, and low frequency tool. They all use small antenna arrays to send electromagnetic waves through the
formation. Traditionally, the measurement of the conductivity or resistivity of a formation has been one of the main surveys performed in a borehole, primarily to determine water saturation. However, a second electrical characteristic of the formation can be determined – the dielectric permittivity. Dielectric devices can be used to determine formation saturation from data dependent on the dielectric permittivity. The basic measurements are of propagation time and attenuation of an electromagnetic wave as it passes through a specific interval of formation. As the propagation time in water is higher than in hydrocarbons or minerals, the measurement is primarily affected by the water-filled pore space of rocks. This is in contrast to the nuclear-porosity tools affected
by the total porosity. In addition, for a wide range of salinities, the propagation time in water is practically constant and so saturation estimations can be made without prior
knowledge of the resistivity of the formation water. When other open-hole log data are available, it is possible to distinguish between oil, gas, and water with unknown changing
water salinity. There are errors associated with conventional porosity tools. The dielectric dispersion logging tool is a relatively new tool that overcomes major problems of preexisting logs. The tool uses electromagnetic wave to detect water-filled porosity.

Muds of different compositions are used in the drilling well process, to support the wall of the borehole along with maintenance of pressure, and to remove rock cuttings generated from the geological formations encountered by the drill bit. The drilling mud invades the formations and modifies the zones surrounding the borehole, mainly, in terms of the physical properties of the rocks, such as porosity and permeability. The identification of this formation damage is important for reservoir characterization, and the subsequent well completion, as well as for the analysis of economic viability. Many years ago, Schlumberger developed a method
for determining mud invasion diameter using the Tornado Chart. Today, practitioners in the oil industry use the Tornado Chart to present geophysical logs. Improving upon Schlumberger’s methodology, Crain used mathematical equations to calculate the mud invasion diameter. In this study, we propose a polynomial mathematical method to determine mud invasion diameter. Our method utilizes the same resistivity well logs, namely dual induction log and dual laterology, though different from that of Schlumberger or Crain methods. The approach developed in this study considers the characteristics of the invasion process while quickly and accurately showing results in the form of a log that can be visualized adjacent to other logs measured in the borehole.

Exploration activity in the Kutch-Saurashtra area has drawn major attention and because of the recent hydrocarbon discoveries offshore Kutch-Saurashtra basin is now moved from category III to II. Accurate understanding of the full present-day stress tensor and pore pressure is critical for field development. In this study, pore pressure, stress magnitudes and horizontal stress direction have been estimated from high resolution geophysical logs and various direct downhole measurements recorded in the studied offshore exploratory wells. Extensive availability of subsurface pressure measurements confirms hydrostatic pore pressure in the tertiary sediments. Magnitudes of vertical, minimum and maximum horizontal stresses indicate a strike slip tectonic stress regime. Wellbore breakouts of A-C qualities have been identified in density image logs and four arm caliper logs, indicating a NNE trend (N10°E to N20°E) of SHMax (maximum horizontal stress). The same has been confirmed from Shear wave anisotropy analysis as well. The interpreted stress direction is in good agreement with the World Stress map data available from the adjacent area. This study is significant in understanding the subsurface behavior during drilling, reservoir modeling, future well stimulation and production optimization.

In a case history from the Mahanadi basin (India), we demonstrate the use of resistivity data from electric-log soundings and from borehole logs to discriminate between saltwater and freshwater aquifers. We use interpreted data from eight surface-based vertical electrical soundings (VES) and electric well logs from three boreholes in this study. We establish a quantitative relation among longitudinal unit conductance S (obtained from VES), water resistivity R w , and layer thickness h. We show that ambiguities in resistivity data interpretation limit its ability to distinguish between freshwater and saltwater aquifers. Electric well-log data interpretation is much more accurate but requires boreholes, which are not cost effective when exploring for groundwater. Integrating well-log-based estimates of R w into resistivity interpretation of surface-based soundings improves its ability to discriminate freshwater aquifers while maintaining cost-effective exploration.

The Masila Basin is an important hydrocarbon province in Yemen, but the origin of hydrocarbons and their generation history are not fully understood. In this regard, 10 crude oils from different petroleum reservoir sections in the Masila Basin were characterized by a variety of biomarker and non-biomarker parameters using GC, GC-MS and stable carbon isotope techniques. Oils from the Masila Basin display pristane/phytane (Pr/Ph) ratios ranging from 1.7 to 2.0, low sulfur content, high C 35 homohopane index, relatively high C 27 sterane concentrations and relatively high tricyclic terpanes suggesting a marine clay source rock that was deposited in mildly anoxic to suboxic conditions with dominantly algal organic matter. C 29 20S/(20S + 20R) steranes and bb/(bb + aa) sterane ratios indicate that the Masila oils have reached peak oil window maturity. Another related feature of these oils is the absence of 18a (H)-oleanane, which suggests a source age older than Cretaceous. The carbon isotope compositions are similar to those of the potential source rocks, which range from À25.4‰ to À28.3‰, indicating a marine environment. The new data presented in this paper suggest that the Masila oils constitute one oil family and that the oil originated from the Upper Jurassic Madbi source rock in the basin.

The electric formation resistivity factor (F) and the seismic compressional wave velocity (Vp) are powerful parameters in understanding the electric and elastic behavior of porous media and in identifying the type of fluid saturating the pore spaces. The formation resistivity factor is a function of various infuences, including
pore and grain properties; saturation, salinity, and viscosity of pore water; formation and pore-water resistivities; cation-exchange capacity; and clay content. The compressional wave velocity is a function of bulk (grain and fluid) density; type of saturant and degree of saturation; and various elastic moduli, including bulk (pore, fluid, and grain) compressibility. Both parameters (F and Vp) are significantly
affected by variations of porosity (¿), pressure, and temperature. The three parameters (F, Vp, Porosity) were obtained from well log measurements for complex, heterogeneous, and consolidated shaly sandstone reservoirs, saturated with multiphase fluids, offshore of the eastern coast of Canada. Relationships among the three parameters,
having coefficients of correlation ranging from 0.75 to 0.92, were obtained. Both F and Vp are correlated inversely to Porosity and directly to each other.

Seismic diffractions may play an important role in seismic interpretation because they characterize geological objects that might not be visible for conventional seismic attribute analysis. Diffractivity may be caused by, and consequently may define, tectonic dislocations (faults and fractures), lithological variations, fluid saturation within rocks.
In this paper, we tie seismic diffractions extracted from vertical seismic profiling (VSP) data and borehole logging, from which we recognize the reasons that are responsible for diffractivity of the strata. First, we process a multi-source multicomponent VSP data set to extract seismic diffractions and construct diffraction images of the strata for all three of the VSP data components. Then, we perform joint analysis of well logs and diffractions to obtain petrophysical attributes associated with diffraction images. We divide the rock succession into several units, which have different diffraction properties. We identify compacted rock, alternating intervals, isolated fractured zones, and fluid-saturated layers.

Geophysical investigation involving seismic refraction tomography (SRT) was conducted at a sewage treatment site located at Ahmadu Bello University Zaria with the objectives of determining the overburden thickness and velocity of the subsurface layers in the area, and delineating any geological structures, such as fracture zone, that may pose a threat to the safe running of the Sewage lagoon system in the area.. Five seismic profiles, with three running N-S and two running E-W, were acquired over the earthen-dikes separating the lagoons with the aid of a 24-channel digital Seismograph and a 5 kg sledge hammer striking a rubber plate. Tomography models were generated for each of the profiles with the aid of ray tracing through a wave-front inversion method that is based on finite difference approximation of the eikonal equation. Geological artifacts, as a result of noise observed in the tomography models of some of the profiles, were removed by filtrations that include manual editing of model's physical parameters and iterative model updating through simultaneous iterative reconstruction techniques (SIRT). The result of the investigation shows that the area consists of three subsurface layers; an undifferentiated overburden layer with a P-wave velocity range of 891 m/s to 1421 m/s, a partially weathered zone with P-wave velocity range of 3010 m/s to 5129 m/s and underlying fresh granitic basement with P-wave velocity range of 5704 m/s to 7762 m/s, while the overburden thickness in the area varies from an average of about 18 m in the northern part to about 55 m in the southern part. The investigation also revealed a fracture zone within the underlying granitic basement, which occurs at about 60 m below the earthen-dike located at the centre of the study area.

The paper presents a numerical algorithm that performs accurate lithology corrections for thermal (CNL/TNPH, CNL/NPHI) and epithermal (SNP) neutron logs, in the whole range of porosities normally encountered in practice. It is known that neutron logs respond not only to a formation’s true porosity, but, to some extent, also to its lithology (the so-called "matrix effect"). Because the matrix effect is nonlinear and varies with true porosity, which is unknown beforehand, an iterative process is needed in order to carry out proper lithology corrections and obtain formation's porosity. The relationships between measured neutron porosity and true porosity for the common reservoir lithologies (sandstones, limestones and dolomites), at different formation water salinities, are approximable via suitable regression polynomials. Nonlinear neutron responses thus expressed are combined with the responses of density and sonic velocity logs, together with the volumetric unity constraint, into a system which is solved using a Newton-Raphson root-finding routine. The algorithm, implemented as a set of Matlab™ codes, provides optimal petrophysical models for the interpretation of neutron-density-sonic log suites and is easily adaptable in order to include additional linear or nonlinear log response equations. A case study regarding its applicability is also presented in the paper.

Borehole instability issues account for approximately 18% of the non-productive time during drilling. The data needed for the planning of optimized wellbore trajectories and mud weight windows can come from multiple sources, notably Logging-While-Drilling (LWD) and wireline measurements. In particular, in-situ tectonic stresses can be inferred from the identification and characterization of breakouts (some of the most common processes that infringe upon the stability of boreholes). Two techniques can help to distinguish the breakouts: (1) determining borehole ovalization and (2) identifying the breakouts on borehole geophysical images. Multi-arm caliper logs from wireline imaging tools provide a direct measure of the borehole size but are not always available. Borehole shape can, however, be inferred from LWD geophysical imaging logs, notably from acoustic and density logs, which are very sensitive to standoff. Synthetic caliper curves can be computed from the standoff curves and converted into an oriented caliper image, displayed as a borehole wireframe. Sections through this borehole wireframe provide ovalization information and eliminate the uncertainty of breakouts direction estimation from resistivity images, which may show sometimes artifacts related to the presence of bad hole conditions or to very high formations true resistivity/mud resistivity ratios.
This paper presents the application of LWD imaging data for better characterization of a borehole cross-section and easier breakouts detection in hostile environments (borehole excavations and rugosity, resistivity image artifacts, presence of fractures and faults). The proposed method enables detecting breakouts in real time or in memory mode. Determining the magnitude and orientation of borehole breakouts enables the validation or adjustment of geomechanical models and the selection of optimal parameters for the drilling mud.

Horizontal wells have been applied in many types of
reservoir conditions with a variety of applications. The
most successful of these has been exploiting the very
large horizontal permeability anisotropies associated with
naturally fractured reservoirs. Log analysis has always
been difficult in naturally fractured reservoirs; very little
has been published regarding horizontal well logs. This
paper summarizes Union Pacific Resources (UPRC)
experience logging dozens of horizontal wells in several
different naturally fractured reservoirs, summarizes the
improved information resulting from this experience,
contrasts various well logs applicable in such areas, and
describes a method for categorizing fracture apertures
used to quantify the total horizontal well fracture index
(RFI).

FOR CITATIONS:
SALEM, H.S., 2000. Fluid-flow characterization of porous media (for the example of the Jeanne d'Arc Basin consolidated reservoirs). Energy Sources. 22(6), July: 557-572.

ABSTRACT:
Flow of fluids in porous media is a complicated process governed by several physical parameters and properties of the solid particles and fluid molecules, as well as the interactions between them. The Kozeny-Carman equation is one of the powerful equations that provides a better understanding of fluid flow. The equation can be
successfully applied to viscous flow and laminar flow through unconsolidated and consolidated porous media if some of its elements are known. One of these elements is the Kozeny-Carman coefficient (Kcc ), which has a specific influence on the
mechanism of fluid flow in porous media. In this study, Kcc , along with other cc parameters, was obtained numerically from well-log measurements for several wells penetrating the Canadian offshore Jeanne d’Arc Basin reservoirs, which consist of consolidated and heterogeneous rocks characterized by a complex network of pores and pore channels. It is shown that Kcc is not a constant but a variable depending on several physical parameters and lithological attributes and strongly related to fluid flow as well as to electric-current conduction in porous media. The Kozeny-Carman
coefficient, obtained as tortuosity (t ) times shape factor (Shf ) agrees well with that obtained as (t) times Archie cementation factor (m). This observation suggests that Shf and m are analogous to each other. An average value of 7.5 was obtained for Kcc , which can be used for similar media instead of the value of 5.0 that is inaccurately used for consolidated porous media. Also, an average value of 3.3 was obtained for (t) and an average value of 2.28 was obtained for Shf (analogous to m). Empirical equations linking Kcc and a variety of petrophysical parameters were also
obtained."

Atlantic shelf seismic structure to the large of Agadir (Southwest Morocco). Offshore seismic reflection and well data have been used to establish the structural style of the offshore of Agadir and to extend the Atlas fabric into the deepwater domain. The obtained structural subsurface map exhibits a complex architecture resulting from the coexistence of normal, inverse, wrench faults and folds cored by Triassic salt. The seismic data indicate a structural style characterised by narrow, detached folding above Triassic salt layers. The strong similarities in the style of deformation between the Agadir offshore and the Cap-Tafelney fold belt, suggest that both of them form one continuous belt, which corresponds to the western termination of the Atlas alpine system. © 2000 Académie des sciences / Éditions scientifiques et médicales Elsevier SAS

Estimation of volume of shale is a crucial step in reservoir characterization. Shale volume is generally estimated using gamma ray logs, as they directly measure shale radioactivity. But it does not mean that other log signatures do not record the presence of shale, and implies that they are not as easily interpretable as done using only gamma ray logs. In this study a methodology using factor analysis and backpropagation neural network is proposed for the estimation of shale volume using gamma ray logs, density logs, and P-wave velocity. Utilization of several logs gives a better estimation of volume of shale and the accuracy of these techniques is discussed and compared with conventional methods.

Laboratory measurements of relative permeability and capillary pressure are seldom performed on core samples retrieved from petroleum-production wells. Reservoir engineers rely on a limited number of small core samples to characterize many of the large-scale multiphase flow petrophysical properties affecting the production and recovery of hydrocarbon fields. The question also remains whether laboratory measurements are truly representative of in-situ rock properties. Non-linear regression methods were recently proposed to estimate saturation-dependent petrophysical properties from fractional flow-rate measurements acquired with formation testers. However, such procedures are still unclear to many practicing analysts and to date have not been fully explored with either synthetic or field data.This paper presents the development and field test of a new method to estimate saturation-dependent rock properties. The two data sets considered in our examples use in-house and commercial reservoir simulators to model the processes of mud-filtrate invasion, acquisition of borehole resistivity measurements, and subsequent fluid withdrawal during formation tester sampling. Capillary pressure and relative permeability are described using the Brooks–Corey model comprised of 6 independent unknown parameters. The iterative procedure repeats itself until measurements are all honored within prescribed error bounds. As a result, the estimation method satisfactorily reconstructs the relative permeability and capillary pressure curves with minimal a-priori information. Whereas relative permeability end-points of water and oil can be readily estimated in a couple of non-linear iterations (assuming that the remaining parameters are fixed), residual and irreducible saturations add complexity to the inversion. We also investigate the use of Design of Experiment (DoE) tools to secure a reliable initial guess for nonlinear inversion and to understand the separate contributions of the various measurements to specific inversion parameters. Such information is fundamental in the design of a data-weighing scheme that selectively enhances the sensitivity of the measurements to unknown parameters during progressive steps of nonlinear inversion.

Nuclear magnetic resonance typically utilizes a tuned resonance circuit with impedance matching to transmit power and receive signal. The efficiency of such a tuned coil is often described in terms of the coil quality factor, Q. However, in field experiments such as in well-logging, the circuit Q can vary dramatically throughout the depth of the wellbore due to temperature or fluid salinity variations. Such variance can result in erroneous setting of NMR circuit parameters (tuning and matching) and subsequent errors in measurements. This paper investigates the use of a non-resonant transmitter to reduce the circuit sensitivity on Q and demonstrates that such circuits can be efficient in delivering power and current to the coil. We also describe a tuned receiver circuit whose resonant frequency can be controlled digitally. Experimental results show that a range of common NMR experiments can be performed with our circuits.The illustration shows a non-resonant NMR transmitter circuit based on four switches. Waveforms used to control the switches are also shown.► We use a non-resonant NMR transmitter to reduce the circuit sensitivity to coil Q. ► We show that this circuit can efficiently deliver current to the NMR coil. ► We describe a digitally-tunable low-noise receiver circuit. ► A range of common NMR measurements has been performed with these circuits.