Well Logging

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.

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.

Практическая ценность этой работы заключается в повышении эффективности разведки нефтяных и газовых месторождений в Восточном Багдаде за счет оптимизации и сокращения комплекса каротажа скважин, отбора керна, отбора проб и испытаний пластов пластов и компьютеризации данных интерпретации для обеспечения необходимого точность и надежность определения петрофизических параметров, которые позволят уточнить и увеличить доказанные запасы месторождений углеводородов в Восточном Багдаде. Для расчета наиболее точных значений водонасыщенности для каждого интервала Зубаирской пласты была разработана конкретная модифицированная форма уравнения Арчи, соответствующая этой формации.

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.

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.

Bottom-hole temperature (BHT) data measured in oil and gas wells constitute a large, lowquality set of temperature observations commonly used in geothermal studies. Raw BHT data are, on the average, cooler than true formation temperatures. To estimate true formation temperatures, corrections must be applied. Empirical BHT corrections may be applied to BHT data for which only depths of measurement are known, but may not be valid outside of the area for which they are calibrated. If multiple BHT measurements from successive logging runs are available, the Horner plot correction procedure can be used. The accuracy of the Horner plot is limited by simplifying assumptions made in its derivation, and by the common lack of information on parameters such as duration of mud circulation. More detailed and complete treatments provide insight into the borehole equilibration process, but their application is similarly limited by a common lack of data regarding borehole thermal properties. A new type of empirical correction procedure may be derived in some areas and allows a correction to be made for BHTs for which only a depth and time of measurement are known. Noise is invariably present in the BHT data found on well log headers from oil and gas wells, and the consequence of noise in these data should be considered when choosing and applying a correction. Although the average nature and magnitude of error in raw BHT data may be approximately known, application of all BHT correction procedures to the typical data available from oil and gas well log headers usually involves an unknown amount of error. Reliable estimates of average geothermal conditions can be made only from suites of BHT data, and are condemned to be imprecise at best.

The Lower Senonian Matulla Formation is well developed in the southern and central Gulf of Suez. It can be subdivided into three depositional units (M1–M3). An obvious unconformity separated the middle unit (M2) from the upper unit (M3). The three units are corresponding to a second-order depositional sequence. This sequence consists of two systems tracts; the lower lowstand systems tract consists of slope fan, followed by a prograding complex. An erosion surface separated the lower from the upper trans-gressive systems tract. The Matulla Formation is mostly composed of sandstones and pelagic sediment intercalations. Such intercalations show obvious facies change throughout the study area, causing further subdivision of the Matulla Formation into seven rock types (RT1–RT7). The petrophys-ical reservoir evaluation has been achieved via determination of the effective porosity, permeability, shale content, water saturation and net-pay thickness from the environmentally corrected well log data. It indicates that the Matulla Formation can be considered as a good reservoir quality especially in its lower unit and sometimes in its middle unit as well. The available core data indicate a lithological heterogene-ity of the studied Matulla Formation. Lithologically, it can be subdivided into four petrophysical and lithological facies; namely, they are sandstones, ferruginous sandstones, argilla-ceous sandstones and limestones. The best petrophysical properties were assigned for the ferruginous sandstones with good to excellent porosity, very good to excellent permeabil-ity, and poor to fair flow zone indicator.

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.

"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."

Logging merupakan metode pengukuran besaran-besaran fisik batuan
reservoir terhadap kedalaman lubang bor. Sesuai dengan tujuan logging yaitu menentukan besaran-besaran fisik batuan reservoir (porositas, saturasi air formasi, ketebalan formasi produktif, lithologi batuan) maka dasar dari logging itu sendiri adalah sifat-sifat fisik atau petrofisik dari batuan reservoir itu sendiri, yaitu sifat listrik, sifat radioaktif, dan sifat rambat suara (gelombang) elastis dari batuan reservoir.

Gamma Ray log known as GR log. Its measure the intensity of natural radioactivity of rocks. The intensity of GR is high in the
Shale. Why?. The answer is very simple. The radioactive particles like Uranium, Potassium, Thorium are very fine grains.
During deposition these particles deposit with shale because Shale is also fine grain rock. That’s why the value of GR is high in
Shale. GR is the best log for correlation.

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.

Cementing in drilling wells is a process of mixing and displacement of the cement slurry through the casing so that it flows up past the annulus behind the casing so that the casing is bound to the formation. Each formation will have different characteristics, because the cement slurry to be used needs to be adjusted in order to meet the standard characteristics of the formation. To be able to design a cement slurry that meets these criteria, laboratory testing must be done regarding the properties of the cement slurry. The characteristics of cement slurry consist of density, thickening time and viscosity, filtration loss, water cement ratio (WCR), waiting on cement (WOC), and compressive strength.

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.

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.

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.

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.

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.

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 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.

Well-Logging adalah suatu upaya pengumpulan data bawah permukaan yang dilakukan dengan merekam data dari sumur bor (Darling 2005). Data yang direkam merupakan properti petrofisika batuan yang berguna untuk melihat kondisi bawah permukaan yang nantinya berkaitan dengan penilaian suatu site. Well-logging juga dikenal sebagai borehole-logging, dimana dari borehole (lubang bor) nantinya data yang dapat di hasilkan berupa data inti bor dan data log geofisika.

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.

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).

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.

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.

Knowledge of the magnitude of in situ stresses is crucial to a broad range of applications, particularly drilling, well completion, and hydraulic fracturing design. Stress field characterization is attained by the determination of magnitude and direction of the three principal stresses: the vertical or overburden stress (S v), the minimum horizontal principal stress (S hmin), and the maximum horizontal principal stress (S Hmax). While numerous direct borehole measurements, such as density logs and leak-off tests, provide reliable assessments of S v and S hmin , estimating local S Hmax remains a challenge. This paper presents a workflow that provides with an approach for estimation and constraining of the maximum horizontal principal stress in a carbonate reservoir from borehole sonic logs, by comparison to the isotropic stress state. Our study was restricted to cases with limited achievable data, namely, conventional logs, which is expected to be the circumstances for most practical applications. The results were compared to published regional stress reports, with the goal of validating and understanding the variations of stresses at the field scale. Those findings were in agreement with the estimations obtained from other acoustoelastic methods and the region fault regime in terms of the relative obtained magnitudes of the stresses. In addition to an increasing trend in the calculated stress magnitudes with depth, we observe local variations of stresses in this oilfield, that we suspected to be caused partially by the non-uniform distribution of production and injection activities. The novelty of our proposed workflow is the ability to give borehole estimates of the maximum horizontal principal stress magnitude for a carbonate field located in a regionally compressive zone, without the need for availability of costly measurements, and by taking advantage of techniques to fill in the gaps of limited data, thus, allowing for integrative use of multiple data types that are commonly available.

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.