Distribution Of Reservoir Rocks Research Articles

How to distinguish between marine and lacustrine sedimentary environments? —A case study of Lishui Sag, East China Sea Shelf Basin

The identification and characterization of marine versus lacustrine sedimentary environments can aid in the assessment of source and reservoir rock distribution, which is important for hydrocarbon exploration. Analysis of sedimentary facies, trace fossils, micropaleontology, geochemical characteristics, and relative water-levels was conducted in this study. Analysis of the core revealed typical sedimentary structures, including fourteen lithofacies and eleven lithofacies associations, which are indicative of wave-influenced origins and tidal processes. The sediments of the Yueguifeng Formation and upper Mingyuefeng Formation were deposited in shallow water settings, which later transitioned into the semi-deep water environments of the Lingfeng Formation and lower Mingyuefeng Formation. Sediments of the lower Mingyuefeng Formation and Lingfeng Formation were deposited in semi-brackish water or brackish water environments, whilst from the lower Mingyuefeng Formation to upper Mingyuefeng Formation and lower Lingfeng Formation to Yueguifeng Formation, the environment was semi-brackish to freshwater. The Lishui Sag experienced a large-scale and rapid relative water-level increase from the Yueguifeng Formation to the lower Mingyuefeng Formation and a decrease during the upper Mingyuefeng Formation. The relative water-level of the Lingfeng Formation and the lower Mingyuefeng Formation was consistent with the inferred eustatic global sea-level, and the relative water-level of the Yueguifeng Formation and the upper Mingyuefeng Formation deviated from the inferred global eustatic sea-level. The typical lithofacies associations, micropaleontology, and geochemical characteristics are identified as the key indicators to determine the sedimentary environments. The sedimentary environment analysis results show that the Lingfeng Formation to the lower Mingyuefeng Formation developed in a marine setting, whereas the Yueguifeng Formation and the upper Mingyuefeng Formation developed in a lacustrine sedimentary environment. The sediments deposited within shallow marine settings form good source rocks, the coastal sediments form good reservoirs, sediments of shallow lacustrine can generate source rock, and the deep-lacustrine sediments form good source rocks that produce waxy crude. Therefore, accurately distinguishing the difference between marine and lacustrine basins is important in terms of the overall understanding of the basin and the hydrocarbon play elements contained within it.

Гетерогенність літогенезу відкладів силуру Волино-Поділля

The paper examines the features of the lithological-lithmological structure and the regime of catagenesis of the Silurian sediments of the Lishchynska and Rava-Ruska sections of Volyno-Podillya. It is shown that in the direction from the southwest to the northeast, the role of carbonate rocks in the composition of the stratum increases, which is connected with the established facies zonation. However, the structure of the section in these areas is different: in Rava-Ruska, it is more thinly layered. The sediments are also characterized by the spatial and age heterogeneity of the distribution of carbonate lithmites: in the first case, they tend to the boundary of the Upper and Lower and the middle of the Upper Silurian, and in the second case, they are developed in the tops of the Lower, in the lower, middle, and upper parts of the Upper Silurian. Attention is drawn to the significant role of clay and the absence of marl formations in the deposits of the Rava-Ruska-1 well, which testifies to the heterogeneity of sedimentation conditions in the mesopelagial of the Silurian basin. The cyclic nature of Silurian sedimentation is established. At the same time, four regressive episodes are recorded in the Lishchynska area, and five in Rava-Ruska, which may indicate a certain specificity of sedimentation conditions in different parts of the basin. The latter directly affects the peculiarity of the spatial-age distribution of reservoir rocks and aquifers. It is shown that the post-sedimentation transformations are mainly related to the development of authigenic silica and calcite, which is found in both clayey and carbonate rocks. A significant difference in the history of the formation of the oil and gas systems of the Lishchynska and Rava-Ruska areas has been established, which allows us to assess their prospects differently. Thus, in the first case, the generation potential of organic matter of Silurian sediments was largely exhausted by the end of the Mesozoic. In the second, large-scale processes of generation and migration of hydrocarbon fluids began only in Paleogene-Neogene time.

The influence of complex palaeobathymetry on development of deep-lacustrine fan systems

Pre-existing complex palaeobathymetry often plays a key role in the spatial-temporal distribution and character of deepwater sedimentary systems. Particularly in deep-marine fan systems where their spatial-temporal association with complex syn-depositional palaeobathymetry have been widely investigated. When present in deep-marine settings, complex palaeobathymetry is known to affect flow-type, flow direction, and resultant fan distribution, which ultimately leads to atypical reservoir rock distribution. By contrast, far fewer studies explore the influencing controls of palaeobathymetry on deep-lacustrine sedimentary systems. This is important to investigate as deep-lacustrine basins have quite different allogenic and autogenic controls on flow types and resultant fan systems and fan lobes, which varies through different stages of basin configuration. To address this knowledge gap, this study documents and characterises a suite of deep-lacustrine sedimentary systems imaged in high-quality 3D seismic data from the rift-sag transitional and early post-rift phases of the North Falkland Basin, Falkland Islands. A range of multi-scalar seismo-geomorphological features are identified, including super systems, fan systems, fan lobes, and channel elements. The influence of palaeobathymetry on flows and resultant sedimentary features is evidenced by frontal and lateral structural confinement at the super system scale, and lateral confinement plus fan/flow deflection at the fan system, fan, and lobe scale. Offset stacking and compensational lobe-scale stacking geometries are developed in response to the type and scale of confinement. Palaeobathymetry, created as depositional relief by preceding fan deposits, is shown to progressively influence flow types and resultant spatial distribution of ensuing sedimentary systems. During periods of basin-fill where encircling palaeobathymetry ultimately controlled super system scale distribution, the ponding of flows and resultant fan features against intra-basinal highs formed thick packages of potentially coarse-grained sediments. As the basin filled-up and encircling topography exerted less control on super system scale distribution, flows were able to surmount the intra-basinal highs, leading to flow stripping processes. The combination of ponding and flow stripping processes resulted in the deposition and preservation of coarse-grained sediments immediately behind or on top of intra-basin structures. The results of this study provide key insights into the interaction of deep-lacustrine sedimentary systems and complex palaeobathymetry, which ultimately influences reservoir distribution.

Calculation of Pore Throat Radius Percentiles (25, 50 & 75) From Porosity and/or Permeability: Algyo Oil and Gas Field, Hungary

Pore throat size distribution of reservoir rocks has a great importance in hydrocarbon migration and entrapment. It is used for study permeability barriers, reservoir characterization and stratigraphic traps. In the present study 51 core samples obtained from Algyo oil and gas field were conducted to MICP laboratory technique to study pore throat size distribution. The inclusive graphical measures of gain size analysis were borrowed for pore throat size examination. Various pore throat radius percentiles such as 25,50 and 75 were calculated and related to both rock porosity and permeability. The obtained models were robust and reliable to use for pore throat radius percentiles (25,50 and 75) calculation. One of these models which is predicting the 50 percentiles was verified. It shows reliable coefficients of correlation (R2 = 0.77 and 0.79) as it is estimated from permeability and porosity, respectively.

Lithogenetic preconditions of reservoirs and reservoir rocks formation in the Middle Devonian sediments of the Skhidnosaratske deposit (Dobrudja Foredeep)

The lithological features of the Middle Devonian sediments of the Skhidnosaratska structure of the Dobrudja Foredeep in the aspect of their influence on the formation of reservoir rocks and hydrocarbon reservoirs have been considered. Nine lithocycles of regressive nature (e-1 – e-5; g-1 – g-4), characterized by a two-membered structure have been identified in the section. Each lithocycle represents a separate productive horizon. The lower parts of the latter are composed of porous and fractured reservoir rocks (limestones, dolomites, siltstones, sandstones), and the upper parts are composed of fluid-resistant packs (marls, anhydrites).Features of cyclic sedimentation conditions caused spatial-age variability of trap localization and morphology, nature of distribution of reservoir rocks of various types, and post-sedimentation processes caused spatial heterogeneity of petrophysical properties of rocks, which entailed different prospects for individual areas of the Skhidnosaratske deposit. The vaulted traps of the productive horizons of the Eifelian deposits tend to the area of borehole 2, and the Zhivetian deposits to the borehole 1. Consequently, in the first case, the most perspective horizons are e-2 and e-3, in the second – g-2 – g-4. Often more powerful packs of better-quality reservoir rocks are developed outside the vault, which leads to the formation of lithological traps in the e-1 and e-2 horizons. It has been shown that the complex structure (presence of layers of carbonate and terrigenous rocks) of fluid-resistant marl-sulfate packs could cause dispersion of hydrocarbons and thus reduce the prospects of individual horizons, in particular, e-2 and e-3. The established heterogeneity of the structure of the productive horizons, the pinching out of individual packs of both pore and fractured reservoirs, the spatial uneven development of the limestones anhydritization and dolomitization processes give possibilities to suggest the formation of separate deposits with different hypsometry of water-oil contacts.

Carbonate seismic sedimentology: A case study of Cambrian Longwangmiao Formation, Gaoshiti-Moxi area, Sichuan Basin, China

This study applied seismic-sedimentological workflow to deeply buried marine carbonate sequences in western China. The workflow aimed at integrating core, wire line log and 3D seismic data to investigate the paleogeomorphology qualitatively and reservoir thickness quantitatively in the carbonate systems. In Gaoshiti-Moxi area of Sichuan Basin, the Longwangmiao Formation was formed in sedimentary environment of carbonate platform facies. Lithofacies vary from dolomitized grainstone to mudstone, which formed shoal, lagoon, and deep shelf sub-facies. The high-porosity (porosity > 4%) reservoir rocks are related to shoal sub-facies. Seismic geomorphology was employed to restore paleostructure of Longwangmiao Formation in the study area by methods of cast and isopach, which were calibrated by well data. It was found that the paleogeomorphology and depositional facies were largely controlled by a syndepositional, en echelon fault system. Following the strategy of seismic lithology, quantitative distribution of reservoir rocks and sedimentary facies was assessed by applying principle component analysis on amplitude attributes generated from multiple frequency panels. Seismic prediction of diagenetic facies (selective dissolution) was achieved by interpreting similarity variance, a seismic geometric attribute. In summary, the procedure, combining seismic geomorphology and seismic lithology, is effective and can be used as a reference for seismic-sedimentological study in other carbonate basins in China and elsewhere.

An outcrop analogue for deepwater salt withdrawal mini-basins: lateral and vertical variations in basin-fill

Mini-basins are well known targets for petroleum exploration as they can contain significant hydrocarbon reserves, such as in the Gulf of Mexico. Though mini-basins have been studied before, their reservoir rock distributions remain poorly predictable. This is especially the case where mini-basins are near salt-diapirs. The Donkey Bore Syncline in the Flinders Ranges, South Australia, presents an excellently exposed deepwater mini-basin reservoir analogue. Detailed outcrop study, including vertical and lateral logged sections presented here, shed considerable light on the depositional system, especially with regards to depositional style and stages of basin-fill. Based on the observed facies and deposit geometries, the authors have been able to detect the change from a carbonate to a siliciclastic deposition system and interpret mini-basin-fill initiation and progradation processes. Sandy gravity-flow deposit lobe areas are identified, enclosed within shales. Along with paleocurrent data, these enable the description and interpretation of basin-fill stages and the lateral character of reservoir-analogue depositional tracts. This will be useful for better interpretation of mini-basin turbidite sequences and will help reduce uncertainties in subsurface exploration. This may also improve prediction of basin-fill where well spacing is inadequate.

Transform margins: development, controls and petroleum systems – an introduction

Abstract This paper provides an overview of the existing knowledge of transform margins including their dynamic development, kinematic development, structural architecture and thermal regime, together with the factors controlling these. This systematic knowledge is used for describing predictive models of various petroleum system concept elements such as source rock, seal rock and reservoir rock distribution, expulsion timing, trapping style and timing, and migration patterns. The paper then introduces individual contributions to this volume and their focus. Supplementary material: Location table and map of specific transform examples, structural elements of the Romanche transform margin and glossary of terms used in this article is available at https://doi.org/10.6084/m9.figshare.c.3276407

Geometry, kinematics and dynamic characteristics of a compound transfer zone: the Dongying anticline, Bohai Bay Basin, eastern China

AbstractThe Dongying anticline is an E-W striking complex fault-bounded block unit which located in the central Dongying Depression, Bohai Bay Basin. The anticline covers an area of approximately 12 km2. The overlying succession, which is mainly composed of Tertiary strata, is cut by normal faults with opposing dips. In terms of the general structure, the study area is located in a compound transfer zone with major bounding faults to the west (Ying 1 fault) and east (Ying -8 and -31 faults). Using three-dimensional seismic data, wireline log and checkshot data, the geometries and kinematics of faults in the transfer zone were studied, and fault displacements were calculated. The results show that when activity on the Ying 1 fault diminished, displacement was transferred to the Ying -8, Ying -31 and secondary faults so that total displacement increased. Dynamic analysis shows that the stress fields in the transfer zone were complex: the northern portion was a left-lateral extensional shear zone, and the southern portion was a right-lateral extensional shear zone. A model of potential hydrocarbon traps in the Dongying transfer zone was constructed based on the above data combined with the observed reservoir rock distribution and the sealing characteristics of the faults. The hydrocarbons were mainly expulsed from Minfeng Sag during deposition periods of Neogene Guantao and Minghuazhen Formations, and migrated along major faults from source kitchens to reservoirs. The secondary faults acted as barriers, resulting in the formation of fault-bound compartments. The high points of the anticline and well-sealed traps near secondary faults are potential targets. This paper provides a reservoir formation model of the low-order transfer zone and can be applied to the hydrocarbon exploration in transfer zones, especially the complex fault block oilfields in eastern China.

Combine the capillary pressure curve data with the porosity to improve the prediction precision of permeability of sandstone reservoir

Abstract Permeability is a critical parameter that reflects the nature of reservoir, playing a crucial role in the development of oil field. It is difficult to accurately calculate the permeability parameter in reservoir evaluation. Capillary pressure curve represents pore-throat size and distribution of reservoir rocks, and as for the porous sandstone, absolute permeability of rock depends primarily on the pore throat distribution. This paper has improved the estimation model for permeability established by Swanson, adding porosity factor, and has successfully established the absolute permeability estimation model for sandstone using Capillary-Parachor parameters. The permeability of 30 rock samples is estimated by using the above-described two kinds of model respectively. The result shows that the permeability estimated by the improved model is in good agreement with the value of permeability of core testing, and has made some improvement in the precision when compared to the permeability estimation model established by Swanson.

A stratigraphic framework for Late Jurassic-Early Cretaceous gas-bearing strata (Monteith Formation) in the subsurface of northwest Alberta

Abstract The entire Late Jurassic–Early Cretaceous stratal package between the Fernie Formation and the Cadomin Formation, from Townships 62–74 in the subsurface of northwestern Alberta, is assigned to the Monteith Formation (Minnes Group). Three mappable units are present, informally termed Monteith A, B and C, from youngest to oldest. The Monteith C was deposited in a marginal marine–deltaic depositional setting. Upward, the transition to coastal plain (Monteith B) and fluvial (Monteith A) depositional settings records a continuous progradation of a long-lived depositional system. Mapping of these units is aided by identification of regionally extensive marine flooding surfaces in the Monteith C, which are used as stratigraphic datums. The thickness of the Monteith Formation decreases from 450 m in the southwestern portion of the study area to a zero-edge in the northeast. The Monteith A, B and C are up to 160 m, 100 m and 200 m thick, respectively. Depositional thinning to the northeast is prevalent, but the primary reason for gross stratal thinning is substantial incision associated with the overlying sub-Cadomin unconformity. The Monteith Formation in Alberta was deposited in a foredeep setting. Regional thickness trends suggest increased accommodation to the southwest and the strata are interpreted to have accumulated on the cratonward side of the foredeep axis. These observations, as well as evidence for basin axial channel flow to the northwest during deposition of the Monteith C, support an interpretation that the foreland basin had initiated prior to Monteith Formation deposition. Basin physiography, as well as uplift and denudation of the Cordillera, affected the depositional evolution of units in the study area. Ultimately, these factors strongly influenced the distribution of reservoir rocks (primarily sandstones of the Monteith A and C), which are part of the Deep Basin Gas System. The presence or absence of permeability enhancing fractures also impacts reservoir quality and well deliverability in the Monteith Formation.

Secondary processes in reservoir rocks of the Yarakta Horizon on the southeastern slope of the Nepa-Botuoba anteclise

Basic regularities of secondary processes in reservoir rocks of the Yarakta Horizon are described with rocks in the Yarakta field as example. To achieve the formulated goal, we elucidated rock types in the studied section, its structure and distribution of reservoir rocks, as well as types of secondary alterations and their influence on filtration-capacity properties. The studies revealed that reservoir rocks of the Yarakta Horizon were formed on an alluvial-deltaic plain on the southeastern slope of the Nepa-Botuoba anteclise. The pore space of reservoir rocks is determined by conditions of their formation and peculiarities of secondary alterations. Maximal filtration-capacity values are characteristic of gritstones, as well as coarse-grained and inequigranular sandstones developed in the lower and middle parts of the Yarakta Horizon. The intergranular space of reservoir rocks in the lower part of the reservoir is substantially “healed” by secondary processes (regeneration, dolomitization, sulfatization, and salinization), probably, due to the gravitational seepage of stratal fluids, mineralization of which increased with time.

Sedimentary Facies and Distribution of Reservoir Rocks from the Feixianguan Formation in the Daxian-Xuanhan Region, NE Sichuan

Abstract The Lower Triassic Feixianguan Formation in the Daxian-Xuanhan region, northeastern Sichuan is interpreted to be a carbonate platform intermediate between the West Hubei-East Chongqing marine basin and Guangyuan-Wangcang marine basin. Outcrops, well logs and seismic data have disclosed that three sedimentary facies can be identified for the formation: open platform, platform exposed shoal and restricted platform facies. During the early stage of deposition of the Feixianguan Formation, there existed a sedimentary framework all the same as the Late Permian one: open platform in the west, and platform exposed shoal, restricted platform and open platform eastwards. The gradual increase of the shoal area permitted eastward migration of the shoal facies. During the deposition of the third member of the Feixianguan Formation, the study area was invaded by temporary transgressions. Till the deposition of the fourth member of the formation, the study area was prevailed over the unified restricted platform deposits, as indicated by the sedimentary model for the typical carbonate platform shoal deposits. The reservoir rocks in the Feixianguan Formation consist mostly of solution opening dolostone, and the rock types are assembled by oolitic dolostone, residual oolitic dolostone and sucrosic residual oolitic medium- to coarse-grained dolostone. The deep and shallow dual laterologs show relatively high resistivity, clear amplitude differences and time difference saltation of acoustic waves in some intervals. The seismic responses are indicated by low-frequency and highly variable amplitudes and chaotic reflection configurations. The reservoir rocks are characterized by high porosity-high permeability and medium porosity-medium permeability, showing a marked correlation between porosity and permeability. Solution openings are extremely developed in the reservoir rocks, including cast pores, intergranular solution openings, intercrystal pores, intercrystal solution openings, solution openings (caves) and minute fissures. Macropores and coarse pore throats are common. The reservoir rocks are apparently controlled by sedimentary facies in distribution patterns. Vertically, they occur dominantly in the second and first members of the Feixianguan Formation, and laterally in the platform exposed shoals and restricted platforms, where good natural gas potential is expected.

An Improvement in Lithology Interpretation from Well Logs in the Patchawarra Formation, Toolachee Field, Cooper Basin, South Australia

Conventional lithology interpretation from well log data becomes unreliable when the target bed is thinner than the vertical resolution limit of the logging tools. The thin bed log analysis method of Bateman (1990), using the concept of binary lithology and a filtering technique applied to conventional logging suites, is able to detect beds as thin as 15 cm. This paper will focus on the use of the Bateman method to improve log-derived lithology estimates in the Permian Patchawarra Formation sandstone gas reservoirs in the Toolachee Field of the southern Cooper Basin in South Australia. In three representative wells, conventional log analysis overlooks most of the thin sandstone beds and underestimates the gross sandstone by up to 36% when compared with core data. In contrast, the thin bed algorithm identifies the thin sandstone beds and gives a gross sandstone thickness which agrees with the core value to within about 5%. It therefore provides a more correct picture of reservoir rock distribution, and reveals more potential pay than conventional log analysis. The method also makes core to log depth matching easier and more accurate. Log character and flow test results imply the presence of relatively permeable gas-bearing zones within some of the thin sandstone beds, suggesting that overlooking thin bed reservoirs may significantly lower reserve estimates. Since the method does not require the use of any high resolution logging device, it is possible to re-evaluate old log data for which conventional log analysis is subject to such under-estimation.

Late Paleozoic plays in East Greenland

A series of Late Paleozoic–Mesozoic basins, which formed as the result of rifting between Greenland and Norway following the Caledonian orogeny, are exposed in East Greenland between 70° and 76°N. The Paleozoic part of the sedimentary basin fill has a cumulative thickness of up to 13 km. The region seems prospective but exploration is still at an early stage without exploration wells. Basic petroleum-related studies of outcrops have been carried out throughout the region, and 1800 km of high-quality multi-fold seismic data have been acquired in Jameson Land between 70° and 72°N. A number of pre-, syn- and post-rift play types are suggested on the basis of inferred source and reservoir rock distribution as well as structural and thermal history. Some of the play concepts have been identified in outcrop, others have been recognized during the seismic interpretation. The principal Upper Paleozoic source rocks include Upper Devonian shales (freshwater), Upper Carboniferous lacustrine shales (freshwater to slightly saline), and Upper Permian marine shales (some carbonate influence). The main reservoir facies are fluvial sandstones in the Upper Devonian to Lower Permian succession and Upper Permian platform and build-up carbonates. Syn-rift Carboniferous plays (mainly structural) and post-rift Upper Permian carbonate plays (essentially stratigraphic) seem most promising. The syn-rift plays have lacustrine shales as source rocks, fluvial Carboniferous to Lower Permian sandstones as reservoirs, and overlying fluvial/lacustrine shales as seal. The post-rift Upper Permian plays have carbonate build-ups and platforms as reservoir targets and the juxtaposed marine shales as source rock and seal.

Interpretation of wireline log and core data from a mid-Jurassic sand/shale sequence

AbstractInterpretation of downhole wireline logs is generally a major source of information used to build up a picture of the quality and distribution of reservoir rocks. Logging tools respond to a number of variables and comparison of a suite of different logs is usually required to isolate and identify the response due to a single factor, e.g. porosity or clay content. A new tool, the litho-density tool (LDT), has been used along with conventional logging tools in a mid-Jurassic sand/shale sequence. The LDT simultaneously measures Pe (effective photoelectric cross-section) and ρa (apparent bulk density). Pe is a function of atomic number which in turn reflects lithology or mineral composition. By using cross-plots derived from the LDT and other logs, patterns of mineral abundance are indicated.

Geology of Tunisia and Adjacent Parts of Algeria and Libya

The southern part of the study area is part of the stable Saharan platform which, since the beginning of the Paleozoic, has been affected only by epeirogenic movements. Pre-Carboniferous Paleozoic strata are largely clastic, both continental and marine. Early invasions of the sea came from the west, but Carboniferous and younger transgressions came from a northerly (Tethyan) direction. Caledonian tectonic movements produced structural alignments and important regressions, but general uplift resulting from the Hercynian orogeny caused the major withdrawal of the sea. The dominant feature of post-Hercynian erosion in Algeria is a great T-shaped anticlinorium which projects into Tunisia. On the east, a projection of the Hercynian Nefusa uplift of Libya extends westward and c nnects with the anticlinorium. Just south of these positive areas is the Erg Oriental basin, which was connected during most of the Paleozoic with the Homra basin of western Libya. The former is bounded in the western part of the study area by the north-northeast-trending Hassi Touareg anticlinorium. Restriction of the western Tethys basin and post-Hercynian subsidence along the margin of the African landmass led to deposition of a thick series of Triassic and Liassic evaporites. Younger strata of the Saharan platform are relatively thin, and although continental sands were important during certain stages, lagoonal dolomite, evaporite, and shale are the dominant rock types. North of the platform was a subsident zone, the Jeffara flexure, where considerably thicker and more marine strata are characteristic. Associated with the flexure and extending beyond it is a complex series of faults. On the north and west, the principal later Mesozoic events consisted largely of epeirogenic uplifts with regression and local erosion, and transgressive downwarps. Frequent and more intense moveme ts characterized the early Cenozoic, and local deformation, marking the onset of Alpine orogeny, began in the Oligocene. Stronger orogenic pulses occurred at the end of the Miocene, but the last and most violent phase of the Tunisian Atlasic orogeny took place during the Pliocene-Pleistocene. In addition to the Saharan platform and Jeffara flexure, Mesozoic-Cenozoic tectonic features include stable shelves of the Pelagian craton in eastern and offshore Tunisia, and of Kasserine Island in central Tunisia. The latter was exposed during latest Cretaceous time and is bounded on the east by a series of north-south-trending faults and Jurassic horsts marking the eastern boundary of the Atlas zones. Between this axis and the Pelagian shelf and between the Saharan platform and Kasserine Island is a series of Cretaceous and Tertiary intracratonic basins, and in northern Tunisia are similar ones containing thick sections of Miocene-Pliocene clastic rocks. In northwestern Tunisia are many Triassic evaporite diapirs, and some thrust faults are present in a zone of very thick Oligocene flysch. Three hydrocarbon-productive trends are present in the study area: (1) Ypresian (early Eocene) carbonate rocks: facies change regionally southwestward from globigerinid mudstone (source beds) to dolomitized nummulitic calcarenite (reservoir) to evaporite (updip seal). Recent discoveries seem to be associated with small structures, but regional extent of the trend and attendant possibilities for stratigraphic entrapment suggest the potential for large reserves. (2) Paleozoic and Triassic sandstones: this trend contains the only proved, major reserves of Tunisia in the Triassic reservoir at El Borma field. Cambrian-Ordovician strata are productive at Rhourde El Baguel in Algeria, and sandstones of the Silurian-Lower Devonian Acacus Formation are capable of production in many Libyan well . Major fields are structurally controlled, but distribution of reservoir rocks is related to Hercynian uplift, which influenced erosion of Cambrian-Ordovician clastic strata and deposition of presalt Triassic detritus. Hydrocarbon source beds for both systems are believed to be Silurian euxinic shales. (3) Cretaceous shoreline: several small oil fields and one gas field currently are productive, mostly from bioclastic carbonate reservoirs of Aptian to early Senonian age. These accumulations are on surface anticlines in a foothills belt which is not deformed so severely as the main Atlas ranges.

Late Quaternary Depositional Processes on Continental Rise of Western Equatorial Atlantic: Comparison with Western North Atlantic and Implications for Reservoir-Rock Distribution

The latest Pleistocene continental-rise sediments of the western Equatorial Atlantic are composed of (1) hemipelagic clay rich in terrigenous silt and organic detritus, and (2) redeposited terrigenous silt/sand beds up to several meters thick. The abundance, thicknesses, structures, textures, and compositions of the redeposited silt/sand beds indicate that the continental rise has been shaped largely by deposition from turbidity currents and related mass flows. In contrast, previous investigators have demonstrated that the continental rise of the western North Atlantic has been shaped largely by deposition from geostrophic contour currents. Thus, although both the Equatorial and North Atlantic rises are on the western side of the ocean basin, receive large influxes of ter igenous sediment, and have similar late Pleistocene sedimentation rates, the two continental rises have very different sedimentary histories. The fact that contour-current deposition has not been a significant factor in shaping the continental rise of the western Equatorial Atlantic indicates that the contour-current model for continental rise formation which was derived from observations in the western North Atlantic cannot be utilized arbitrarily to explain formation of all continental-rise sequences, ancient or modern. The process which formed any particular continental rise may have an important bearing on the potential of that rise as a hydrocarbon province. Continental-rise sequences shaped by turbidity-current processes (such as the western Equatorial Atlantic) would seem to conta n both excellent source and reservoir rock: whereas good reservoir rocks apparently would be absent from continental-rise sequences shaped by contour currents (such as the western North Atlantic).

Pore Size Distribution of Petroleum Reservoir Rocks

Abstract An investigation of equivalent pore entry radii in typical samples of petroleumreservoir rock and the pore volume associated with each value of pore entryradius has been made. Theoretical discussions together with experimentalprocedures for obtaining pore entry radii and the distribution of pore volumewith pore entry radii are presented. Experimental results on a number of coresamples, along with typical distribution curves, are shown. Data on the percent of the pore volume filled by mercury at a pressure of 1500 psi areincluded. The results of re-runs of samples, made possible by regenerativeapparatus, show the repeatability of the data and indicate the amount ofphysical change in the samples by mercury penetration. Theoretical equationsfor calculating permeability from pore size distribution data were derived andthe results of such calculations are compared with measured gas permeabilitydata. The effect of the shape, surface area, and weight of the sample on thepore size distribution of reservoir rocks was investigated experimentally.Mercury capillary pressure curves are compared with those obtained by theporous diaphragm method using gas to displace water. Introduction Standard core analysis techniques, in general, lead to quantities which arestatistical averages of the varying properties of the samples underexamination. Although such statistical information has real value in predictingthe gross performance of porous bodies, it fails to provide fundamentalinformation concerning the properties of the porous medium itself or anyprocesses which may be occurring in it. For example, a gas permeabilitymeasurement on a core sample is an indirect way of determining an average poreradius for that particular sample. Since there are many combinations of poreradii that will give the same radius, and, hence, the same permeability, noinformation is obtained on the pore size distribution. Furthermore, similarvalues in permeability do not imply similarity in other properties of a porousmedium. For instance, it is possible to have two core samples with identicalpermeability which would have different residual oil contents at the end of anair-oil drive, or even different amounts of interstitial water. It has been recognized for some time by the petroleum industry that adetermination of pore size distribution for porous reservoir rocks offeredpromise of increased understanding of fundamental flow processes in the porousmatrix, and therefore of petroleum reservoir performance in general. T.P. 2893