Marine Bar Research Articles

Evolution of Marine Bars under Micro and Mesotidal Settings of Mahanadi Delta Coast

Shoreward migration process of longshore bar is an important aspect of shoreline changes in the Mahanadi delta front. It involves coalescence of individual bars to form large complex bars just before welding on to the shoreline. Such welding sometimes results in the formation of large hook spits. Bar welding mechanism can be an extremely important form of natural beach nourishment. This study aims to detect stages of evolution of longshore bars and consequent welding with mainland. Multi-dated Landsat images have been used to detect evolution of longshore bar and welding processes. QGIS platform has been used to process images and compose required maps.

Application of IUCN Red List of Ecosystems to assess the ecological status of marine bar ecosystems of Burullus wetland: A Ramsar site

The IUCN Red List of Ecosystems has been employed to assess the status of ecosystems and evaluate the risk of their collapse. The RLE assessment was conducted on the main ecosystems of the marine bar of Burullus wetland; sand plain and salt marshes ecosystems of the marine bar which separates the lake from the Mediterranean Sea. Four criteria were used for conducting the assessment; (A) reduction in distribution, (B) constricted distribution, (C) abiotic components degradation, and (D) disruption of biotic processes and interactions. The integration of remotely-sensed data allowed the application of RLE to study the changes in ecosystems distribution for Criteria A and B. Criterion A showed the same result for sand plain and salt marshes as Critically Endangered (CR). This indicates the distribution of the marine bar is extremely affected by the urban and human activities in the region. It is also expected to be affected by any slight changes in the sea-level and costal erosions in the future in terms of disruption in biotic processes and interactions. Criteria D was applied using floristic quality assessment for current floristic composition and the available historic data. The results show that sand plain and salt marshes are Near Threatened (NT). However, the application of Criteria C was Data Deficient DD due to lack of long term data about the abiotic variables responsible for environmental degradation. The identical results of RLE assessment of sand plain and salt marshes reflect that both ecosystems are equally affected by the ongoing activities in the region, particularly over the last few years. The results indicate that the marine bar ecosystems were disturbed by anthropogenic events such as urban development, grazing and farming. Also natural events such as climate change and its consequences are expected to have severe impacts on the marine bar in the future. Therefore, further studies should focus on the study of the main interactions and processes in the marine bar ecosystems.

Seismic response to paleo-sand dunes in the Nugget Sandstone Formation, southwestern Wyoming

We have analyzed a 3D seismic survey acquired for a carbon sequestration project on top of the Moxa Arch in southwestern Wyoming. We observed a zone of discontinuous reflectors on vertical slices of seismic amplitude volume, whereas, the northwest–southeast lineations were observed on the time slices. We performed a seismic to well tie that suggested that the lineations occur within the Nugget Sandstone. The Nugget Sandstone is an eolian sandstone deposit of Early Jurassic age, deposited as a subtropical dune field, and equivalent to the Navajo Sandstone of southwestern Utah. Petrophysical analysis indicates that the Nugget Sandstone is dominated by clean sandstone (70%–80%), whereas evaporites, including halite and anhydrite, are present in certain zones. Previous outcrop studies on the Navajo Sandstone indicate the wind direction to be predominantly northeast–southwest. Seismic attributes, including coherence and curvature, displayed on stratal slices within the Nugget Sandstone interval indicate the presence of lineations in the northwest–southeast direction with irregular spacing. These lineations are approximately perpendicular to the inferred dominant wind direction. We computed the dominant wind direction from the average azimuth of the lineations as seen on the curvature attribute in the Nugget Sandstone interval. Geological feature: Eolian sand dunes with interdunal evaporites Seismic appearance: Parallel lineations with irregular spacing on seismic attribute horizon slices Alternative interpretations: Canyons at continental slopes; slope failures Features with a similar appearance: Marine bars; contourites Formation: The Nugget Sandstone — equivalent to the Navajo Sandstone Age: Early Jurassic Location: Moxa Arch, Wyoming Seismic data: Obtained by the University of Wyoming with U.S. DOE funding Contributors: Dhruv Agrawal, Brady Lujan, Sumit Verma, Shuvajit Bhattacharya, and Subhashis Mallick Analysis tools: Coherence and curvature attributes; seismic inversion; petrophysical inversion

Nanopetrophysical characterization of the Mancos Shale Formation in the San Juan Basin of northwestern New Mexico, USA

The Mancos Shale of the San Juan Basin has been an important resource for the exploration and development of oil and gas. However, as with most unconventional plays, the shales have low porosity and extremely low permeability with predominantly nanometer-sized pores. Thus, it is critical to understand the nanopetrophysical properties of the reservoir so a proper assessment of the reservoir quality can be made. Working with three as-received core samples from three different wells of the Tacito Marine Bar and Offshore Mancos play types, we have developed an initial understanding of the nanopetrophysical properties of the pore structure as well as fluid-rock interactions in these tight reservoirs. We have performed a suite of integrated tests, such as mercury intrusion porosimetry (MIP), low-pressure nitrogen physisorption, core plug porosity and permeability, scanning electron microscopy imaging, water immersion porosimetry after vacuum pulling, contact angle, and fluid imbibition. In addition, we obtained supplementary data for total organic carbon, X-ray diffraction, and pyrolysis to further evaluate reservoir quality. The Mancos Shale samples exhibit petrophysical characteristics that are controlled by a predominant presence of nanometer-sized pore space, with 56%–96% pore volumes present as 3.4–50 nm in pore-throat sizes, as shown by the MIP approach. Contact angle and fluid imbibition tests demonstrate that samples are oil wet to mixed wet, with a tendency of pore networks to imbibe oil over water. The findings from integrated pore structure and wettability studies provide a database and some insights, from the perspectives of nanopetrophysical characterization, into the reservoir quality of the Mancos Shale.

Satellite images survey for the identification of the coastal sedimentary system changes and associated vulnerability along the western bay of the Gulf of Tunis (northern Africa)

Abstract. The aim of this study consists in testing the effectiveness of satellite data in order to monitoring shoreline and sedimentary features changes, especially the rapidly changing of Gulf of Tunis coast. The study area is located in the Gulf of Tunis western bay (Southern Mediterranean Sea) which is characterized by sandy beaches of Ghar Melah and Raoued (Medjerda Delta area). The aerial photographs and satellite imageries were used for mapping the evolution of shoreline. Diachronic data (satellite imagery, aerial photography and topographic maps) were used to monitor and to quantify, the evolution of the coastal areas. These thematic data were digitally overlaid and vectorised for highlighting the shoreline changes between 1936 and 2016, in order to map the rate of erosion and accretion along the shoreline. Results show that the accretion and degradation are related to the Medjerda: change of outlet in 1973 and impoundment of the Sidi Salem dam in 1982. We found that the general trend of the coastal geomorphic processes can be monitored with satellite imageries (such as Sentinel A2, Spots 4 and 5), due to its repetitive coverage along the time and their high quality concerning the spectral contrast between land and sea areas. Improved satellite imageries with high resolution should be a valuable tool for complementing traditional methods for mapping and assessing the sedimentary structures (such as shoreline, delta, marine bars), and monitoring especially the lowlands coastal areas (slightly eroded).

Storms Consolidated Field, Illinois Basin: Identifying New Reserves in a Mature Area (E & P Notes)

Discovered in 1939, Storms Consolidated field in the central Illinois basin has produced approximately 24.5 million bbl oil, mainly from Mississippian sandstone reservoirs, yet may contain significant remaining reserves. The field produces from a combination structural-stratigraphic trap along the upthrown side of a major normal fault trend. Although the principal reservoir zone, the Waltersburg Sandstone, is mostly depleted, another important interval, the Aux Vases Sandstone, appears underexploited in many parts of the field. Description and analysis of the only continuous core through the Aux Vases has led to the identification of major lithofacies and their characterization in terms of reservoir quality. The Aux Vases appears to represent a series of shallow marine bars separated by nonreservoir areas of siltstone, shale, and shaly sandstone. Log-based mapping of reservoir distribution, plus production and pressure data, indicates significant lateral and vertical compartmentalization of the reservoir. Although insufficient data exist for full-scale integrated reservoir characterization studies, a trend surface analysis was performed on structural information across the field. The results of this analysis were compared with reservoir distribution maps to identify areas for future drilling or recompletion.

Oscillating Quaternary water levels of the Marmara Sea and vigorous outflow into the Aegean Sea from the Marmara Sea–Black Sea drainage corridor

Detailed interpretation of single-channel air-gun and deep-tow boomer profiles demonstrates that the Marmara Sea, Turkey, experienced small-amplitude (∼70 m) fluctuations in sea level during the later Quaternary, limited in magnitude by the sill depth of the Strait of Dardanelles. Moderate subsidence along the southern shelf and Quaternary glacio-eustatic sea-level variations created several stacked deltaic successions, separated by major shelf-crossing unconformities, which developed during the transitions from global glacial to interglacial periods. Near the Strait of Dardanelles, a series of sand-prone deposits are identified beneath an uppermost (Holocene) transparent mud drape. The sandy deposits thicken into mounds with the morphology and cross-sectional geometries of barrier islands, sand waves, and current-generated marine bars. All cross-stratification indicates unidirectional flow towards the Dardanelles prior to the deposition of the transparent drape which began ∼7000 years BP, in strong support of the notion that the Marmara Sea flowed westwards into the Aegean Sea through the Dardanelles at times of deglaciation in northern Europe. The global sea-level curve shows that, at ∼11,000 and ∼9500 years BP, sea level rose to the sill depths of the Straits of Dardanelles and Bosphorus, respectively. The effect from ∼11,000 to ∼9500 years BP was seawater incursion into the Marmara Sea, drowning and formation of algal-serpulid bioherms atop lowstand barrier islands, and transgression of shelves and lowstand deltas. At ∼9500 years BP, glacial meltwater temporarily stored in the Black Sea lake, developed into a vigorous southward flow toward the Aegean Sea, forming west-directed sandy bedforms in the western Marmara Sea and initiating deposition of sapropel S1 in the Aegean Sea. This strong outflow persisted until ∼7000 years BP, after which a mud drape began to accumulate in the Marmara Sea and euryhaline Mediterranean mollusks successfully migrated into a progressively more saline Black Sea where sapropel deposition began. Most eastern Mediterranean sapropels from S1 to S11 appear to correlate with periods of rising sea level and breaching, or near-breaching, of the Bosphorus sill. These events are believed to coincide with times of vigorous outflow of low-salinity (?fresh) surface waters transiting the Black Sea–Marmara Sea corridor, and ultimately derived from melting of northern European ice sheets.

PROSPECTIVITY OF THE OFFSHORE SYDNEY BASIN: A NEW PERSPECTIVE

Approximately 40 per cent of the 52,000 km2 Sydney Basin lies in shallow waters (less than 200 m) off the central New South Wales coast. Containing more than 5,000 m of Permo-Triassic marine and non-marine sediments, and having been the subject of several previous exploration campaigns, no wells have been drilled in the offshore despite widespread numerous occurrences of oil and gas onshore.The Sydney Basin, together with the Bowen and Gunnedah basins, form a major longitudinal Permo-Triassic basinal complex stretching 2,500 km down the eastern margin of Australia. Whereas the onset of this basinal development may have been extensional, reinterpretation of seismic and other geophysical data highlight the potential role played in the early development of the Sydney Basin by easterly directed compression. A compressional style is to be contrasted with the dominantly extensional style interpreted by others for the adjacent onshore areas. The most conspicuous structural element in the offshore, the Offshore Uplift, is interpreted to represent the western overthrust edge of the Currarong Orogen. Accepting the Panthalassan margin geometry of Veevers and Powell (1994) it follows that the Offshore Uplift and restored Dampier Ridge would have constituted a 'greater Currarong Orogen'. A series of progressive westerly directed thrust fronts may have been established across the Panthalassan margin, including the uplifted western margin of the Currarong Orogen, which over-rode and created a thrust load onto the eastern margin of the Lachlan Fold Belt. Much of the Early Permian development of the Sydney Basin therefore could have resulted as a consequence of foreland loading. This is consistent with depositional trends including the overall westerly directed marine transgression which dominated the sedimentary record of the Early Permian. Alternatively, this marine transgression may represent the sag phase induced along a segment of the Bowen-Sydney rift system that had been offset by the Hunter River Transverse Zone from the Gunnedah Basin to a site coincident with the Offshore Syncline.Previous interpretations identified structural development of the Currarong Orogen as either a Cretaceous (Tasman Sea rift related) or Middle to Late Permian phenomena. Early Permian structural growth of the offshore Uplift has important implications for petroleum exploration. The major impediment to exploration appears to be the perception that the Sydney Basin lacks suitable reservoir targets and is gas-prone. Potential source and seal sequences occur extensively within both Early Permian marine shales and siltstones and Early and Late Permian coal measure sequences. The emerging uplift provided a major sediment provenance area and represented a barrier behind which restricted anoxic conditions flourished, conditions favouring the preservation of organic matter. Late Permian and Triassic sequences are absent across the crestal portions of the uplift. However, the emerging, sea-ward facing flank of the uplift would have been subject to marginal and shallow marine, wave-base, barrier and strand bar deposition during the Lower Permian, conditions known in the onshore to favour better reservoir development.Gas demand to the greater Sydney region is anticipated to exceed supply by the year 2000, and new gas markets are being eagerly sought in time for the expiration, in 2006, of the current contract under which gas is supplied to Sydney via the Moomba pipeline.Cretaceous, Tasman Sea rift related, structuring is subordinate to that of the earlier compressional and wrench related structuring. Several new structural targets have been added to the existing inventory of prospects and leads, including some now considered optiminally located with respect to source rock and reservoir development.

Stratigraphic Sequences and Reservoir Facies of the Wapanucka and Spiro Formations: ABSTRACT

An understanding of the stratigraphic boundaries and lithofacies of the Spiro, middle shale (sub-Spiro shale in the subsurface), and the Wapanucka, will lead to a better prediction of the location of petroleum reservoirs in southeastern Oklahoma. Although the Spiro rests unconformably on the Wapanucka in some parts of the Arkoma basin; the base of the Spiro and the top of the Wapanucka are conformable surfaces throughout the frontal Ouachita outcrops. Small, discontinuous deposits of cross-bedded sandy calcarenites and bioclastic sandstones commonly occur within the middle shale sequence. These are commonly referred to as the `sub-Spiro sand` in the subsurface, and can make it difficult to distinguish the base of the Spiro and top of the Wapanucka sequences. At the Wapanucka shelf margin, subtidal subtidal cycles are capped by shoaling carbonate sand bodies (bioclastic and oolitic grainstones). In areas of paleotopographic highs, these shoal deposits can build up above sea level and form island beaches. Subaerial exposure is evidenced by the abundance of trees that grew on these islands. Sponge boundstones and phylloid algal bioherms grew adjacent to the shelf margin, while tubular algal (Donezella) boundstones occasionally formed small banks in slightly deeper water. The Spiro contains identical lithofacies to themore » Wapanucka; however, in certain areas, significant amounts of sand were transported to form shallow marine bars. In addition to the sandstones of the Spiro, several limestones facies of the Wapanucka and Spiro are potential hydrocarbon reservoir rocks. Fracture styles, related to certain lithofacies in outcrop, can aid in predicting fractured reservoirs in the subsurface.« less

Restudy of the Sedimentary Environment of Los Jabillos Formation of Northeastern Venezuela

From the study of five sections located on the southern flank of the Interior Range, we have determined the sedimentary characteristics of Los Jabillos Formation and propose a model for its deposition. During Oligocene time there were changes in the paleoslope direction of the basin, with a predominance of southwest to northeast. The relative variations of sea level, in this part of the Western Venezuelan basin were controlled by subsidence tectonic, which was of major influence in the sedimentation of Los Jabillos Formation. During this epoch and under these conditions, the shelf was covered by sediments from Guayana shield. There were deposits in a wave dominated delta, where the stream-mouth bars were continually reworked into a series of superimposed coastal bars. The littoral currents transported these sands sediments parallel to the coast, causing a decrease in thickness of the units both toward the east and the west. The sandy beds (beach and marine bar), deposits on both sides of stream mouth are preserved in outcrops in Querecual, Oro, and Areo rivers. They are composed of mature quartz sandstone, with good size selection and have, therefore, good permeability. Examples of the marine bar facies have also been studied in wells inmore » the Maturin subbasin where data were obtained from a conventional core.« less

Recent Horizontal Drilling in Lockhart Crossing Field, Livingston Parish, Louisiana: Its impact on Geological Interpretation

ABSTRACT Recent horizontal completions drilled by Callon Petroleum Company in the Lockhart Crossing Field have further optimized production in the Lower Eocene First Wilcox Sandstone, a marine bar composed of 40-45 feet of very fine to fine-grained glauconitic sandstone. This reservoir has produced a total of 16 MMBO and 17 BCFG from 40 wells since 1982. Well site geology was a key factor in these successful horizontal completions because these wellbores were not mudlogged; were not electrically logged; and cores were not taken. Over 2,800 feet of drill cuttings, taken at 10 foot intervals, were examined to maintain the drilling wellbore near the top of the sandstone where the highest values of permeability and porosity are present. By drilling horizontally, First Wilcox Sandstone structure was encountered that was not previously mapped using existing subsurface well control. Callon's International Paper Company (IPCO) #6 well, the first of the three horizontal wells drilled in the Lockhart Crossing Field, flowed at a rate of 527 BOPD and 400 MCFGPD on a 15.5/64 choke with a flowing tubing pressure of 650 psi from the First Wilcox Sandstone in March of 1992. Current flowrates continue to average 600+ BOPD on a 32/64 choke. This well, located upthrown to a down-to-the-south fault, has produced over 227 MBO and should adequately drain this area of the reservoir. A second horizontal well, the IPCO #15, was sidetracked as a horizontal injector and is located near the geographical center of the field. The third and northernmost horizontal well in the field, the IPCO #7, has produced in excess of 47 MBO and continues to produce at rates in excess of 600 BOPD on choke.

Descripción y génesis de los bancos de oolitas ferruginosas en la base del Silúrico de la precordillera Argentina

Detailed outcrop and microscopic descriptions and current ideas about factors controlling the origin of oolitic ironstones coming from the Lower Silurian of the western Argentina Precordillera Basin are given. Chamosite and goethite comprise around 20 to 98 % of the oolitic ironstones. Phosphatic nodules are common components in these deposits. Lithofacies arrangement, outcrop architecture and bioturbation patterns allow to interpret them as shallow marine bars, which typically constitute parasequences boundaries. They are associated with widespread detrital facies. Their origin is related to sea-level fluctuations and a good correlation with global events can be established. The origin and distribution of the ironstones in the Precordillera basin is related with cool-water seas in high southern latitudes, during a high stand of sea-level, occurring after the Late Ordovician glacial event.

Late Pleistocene stratigraphy and palaeobotany of the Isles of Scilly

A re-evaluation of the Pleistocene stratigraphy of the Isles of Scilly has enabled the formal definition of eight lithostratigraphic units of member status grouped into two formations. A chronology of events has been provided by radiocarbon (14C) determinations, optical and thermoluminescence (TL) dates. Inter-site correlations have been strengthened by palynology, which has aided palaeoenvironmental reconstruction. The defined units have been incorporated into two lithostratigraphic models, one for the ‘northern’ (glacial) Scillies and one for the ‘southern’ (extra-glacial) Scillies. Raised beach sediments of the Watermill Sands and Gravel in the southern Scillies are overlain by the Porthloo Breccia, a unit of soliflucted material derived exclusively from the weathering of local granite. Organic sequences at Carn Morval, Watermill Cove, Porth Askin, Porth Seal and Bread and Cheese Cove occur within the Porthloo Breccia, and are interpreted as the infillings of ponds associated with active solifluction. Radiocarbon determinations from these organic sediments are critical because they pre-date units associated with a glacial event. The14C determinations indicate deposition of the organic material between 34 500+885-800(Q-2410) and 21 500+890-800(Q-2358) years BP and provide a maximum age for the glacial event and the first radiometric dates for the coastal ‘head’ sediments of southwest England. The pollen assemblages from these organic sites all record open grassland vegetation, and represent the earliest vegetational record for the Scillies. HighPinusvalues are interpreted as evidence of climatic deterioration. In the southern Scillies, the Porthloo Breccia is overlain by the Old Man Sandloess, a coarse aeolian silt with subdominant fine sand, TL-dated to 18 600+3700-3700years (QTL -1 d and 1 f; Wintle 1981) and optically dated to 20 000+7000-7000and 26 000+10 000-9000years (two samples; 738al and 741al; Smithel al. 1990). This material occurs in a variety of facies related to different modes of reworking. In the northern Scillies, the Porthloo Breccia is overlain by three units that are all related to a single glacial event. The Scilly Till, a massive, poorly sorted, clay-rich pale brown diamicton containing abundant striated and faceted erratics of northern derivation, occurs at Bread and Cheese Cove, and Pernagie and White Island Bars. This sediment is of uncertain depositional facies, although the available data suggest that it may be a lodgement till. At Bread and Cheese Cove the Scilly Till occurs in association with a matrix-supported sandy gravel, the Tregarthen Gravel, which has an erratic assemblage consistent with the underlying Till. The distributional relation between the glacially derived sediments, marine bars and morphological varieties of granite tors suggest that some of the bars may be remnant moraines, and that the glacier was erosive in the northern Scillies. Aeolian loessic processes in association with the glacial advance resulted in the deposition of the Old Man Sandloess in the southern Scillies. The relative coarseness of this material is interpreted as a function of its proximity to glacially derived source material. The mineralogy of the Scilly Till is sufficiently similar to the Old Man Sandloess to suggest a genetic link between the two units. Overlying the Scilly Till and Tregarthen Gravel in the northern Scillies is the Hell Bay Gravel, an extremely widespread matrix-supported gravel containing a similar assemblage of striated and faceted erratics to the underlying Till, but alongside a considerable proportion of locally derived granitic material. The matrix of the Hell Bay Gravel is identical to colluvially reworked facies of the Old Man Sandloess. This material represents an initial phase of solifluction, post-dating the glacial event in which the Scilly Till, Tregarthen Gravel and Old Man Sandloess were mixed and transported downslope. In situations where these sediments were stripped from the land surface, weathered granite once again became the dominant raw material for solifluction, this subsequent phase being represented by the Bread and Cheese Breccia in the northern Scillies and the upper Porthloo Breccia in the southern Scillies. The evidence therefore suggests that ice advanced at least as far as the northern Isles of Scilly during the Dimlington Stadial of the late Devensian Substage. This conflicts with previous interpretations which place the glacial deposits within the Wolstonian Stage. However, the late Devensian event was probably not the first glacial event to have influenced the Islands because erratics are widespread in some exposures of the Watermill Sands and Gravel; the age of this earlier event remains uncertain.

Reservoir Characterization of Multiple-Bar Sandstones in the Mississippian Cypress Formation, Tamaroa Field, Perry County, Illinois

Tamaroa field produces principally from lenticular sandstones in the upper part of the Chesterian Cypress Formation. These sandstone bodies, interpreted to be marine bars, are typically less than 10 ft thick, 0.25 to 0.5 mi wide,and less than 2 mi long. The bars are vertically stacked and trend northeast-southwest in an en echelon pattern. Spontaneous potential and gamma ray log character, as well as core data, show that shales ranging in thickness from 10 ft to less than 1 ft separate the sandstones. Hydrocarbons are produced from the clean parts of the bars that drape across three structural folds. Multiple structural closures and the stacking of shale-separated bars create discrete reservoir compartments in Tamaroa field. Understanding reservoir compartmentalization is a key factor in designing and infill drilling or waterflood program in a multiple-bar complex such as the one at Tamaroa field. Optimum recovery efficiency therefore can be achieved by using a detailed reservoir model.

Palynostratigraphy and palynofacies reconstruction of the Upper Jurassic to Lowermost Cretaceous of the Draugen Field, offshore Mid Norway

In 1984 Norske Shell discovered the Draugen Field with well 6407/9-1 at the edge of the Trøndelag Platform, offshore Mid Norway. The discovery was made in the offshore marine bar sands of the Upper Jurassic Rogn Formation. Based on the palynological assemblages observed in the Rogn and the overlying Spekk Formation four zones and seven subzones can be recognized. They range in age from Mid Oxfordian to Late Ryazanian. Furthermore, four palynofacies have been recognized, characteristic of different offshore marine environments: from lower offshore with stagnant bottom conditions to upper bar washover deposits. Integration of palynofacies data with the stratigraphic framework shows that in the Mid Oxfordian to Early Kimmeridgian the highest energy and the source area lay to the south and the southwest. During the later part of the Kimmeridgian the highest energy sequences (upper bar with washover deposits) are found in the north-central and south of the area. During the Portlandian to Late Ryazanian waterdepth increased rapidly resulting in a lower offshore environment with stagnant bottom conditions. From the depositional history it may be concluded that the Rogn bar could develop in the Draugen area because the uplifted faulted margin of the Trøndelag Platform created the right energy conditions for development and preservation of the bar.

DEPOSITIONAL HISTORY OF THE RESERVOIR SANDSTONES, AKPOR AND APARA OILFIELDS, EASTERN NIGER DELTA, NIGERIA

A subsurface study of the Agbada Formation in the Akpor and Apara oilfields in the eastern Niger Delta was undertaken in order to determine the depositional development of the reservoir sand‐bodies.In the study area, the formation comprises a maximum of 29 relatively thick sandstones, cyclically interbedded with shales of varying thicknesses. The lowest tow sand units, separated by a shale unit and located near the base, produce petroleium from ten wells in both fields. Each sand‐body exhibits a coarsening‐upward textural gradient.The lower sand‐body (Unit A), which is oval‐shpaed, moderately‐ to poorly‐sorted, fine‐ to very coarse‐grained, carbonaceous, with clay and lignite horizons, is interpreted as a deltaic sand. The unit appears to have migrated landwards (north) (destructive delta), probably as a result of localized transgression, and developed maximum thickness between the two fields.The upper sand (Unit B) is fine‐to‐pebbly in grain size, well‐sorted and glauconitic. Maximum development occurred slightly east of the delat. As the unit prograded in a predominantly southerly direction, genetic currents simultenously caused it to be channelized. This sand‐body is interpreted as an offshore marine bar. The intervening carbonacous shale is thought to be of logoonal origin.Since the geometries of these sand‐bodies are not conclusively delimited by the existing well density, their pinch‐out edges to the north, east and west may constitute prolific stratigraphic traps.

Effect of Fractures on Reserve Calculations as Determined by Petrology: Birthright Field, Hopkins County, Texas: ABSTRACT

ABSTRACT Birthright field in Hopkins County, Texas produces oil and gas from Jurassic carbonate grainstone deposits of the upper Smackover Formation. The field was discovered in 1965 by the drilling of the well Schneider and Corey #1 E. M. Strode. This well initially tested 491 BOPD from a 34 ft interval at 9519 ft. The field has an oil-water contact at subsea 9090 ft and a gas-oil contact at subsea 8980 ft. Ten wells were originally drilled in the 800 acre field but only 6 oil and 1 gas wells were put on production. The estimated inplace reserves in 1969 were 6.4 MMBO, 1.3 MMBC, and 14.97 BCF, with an estimated primary recovery of 22%. The total cumulative oil production to 1986 was 3.19 MMBO. The grainstones of the producing interval are composed of ooids, pisoids, pelecypod fragments, grapestones, and fecal pellets. This shallow marine bar deposit has been extensively leached and cemented. Fracturing and minor brecciation has taken place causing many grains to be broken and sheared. Many grains have been coated by early diagenetic fibrous rim-cement indicative of leaching in vadose and phreatic conditions. These conditions have caused significant variations in the structural conditions and sizes of the coated grains. Anhydrite and sparry calcite has subsequently filled many of the pores, especially below the oil-water contact. It is thought that these factors may have affected actual recoverable reserves, and that petrology can be an effective tool in reservoir evaluation especially where recovered reserves are greater than had been initially calculated.

Unconformities and Valley-Fill Sequences--Key to Understanding Reservoirs at Lonetree and Poncho Fields, Denver Basin, Colorado: ABSTRACT

Previous interpretations suggested that the Lower Cretaceous ''J'' sandstone in the Denver basin is divisible into three genetic units: a lower delta front (J-3), a middle delta plain (J-2), and an upper destructional marine bar (J-1). The presence of root casts and siderite cement along the upper contact of the J-3 sandstone, however, suggests that J-2 deposits are separated from the J-3 sandstone by a regional unconformity. This contact may mark the same unconformity that separates the Horsetooth and Fort Collins Members in outcrop along the western margin of the basin. J-2 deposits comprise point bar, crevasse splay, and floodplain sequences of a broad alluvial plain. Fluvial deposits at Lonetree and Poncho probably formed in a tributary to a larger order tributary that flowed southwest through the Peoria field area. These tributaries eventually drained into a major trunk stream represented by valley-fill deposits of the Kassler sandstone. Delta-front deposits (J-3) that underlie the unconformity are tightly cemented by siderite and do not produce in either field. J-2 sandstones are productive from north- and west-trending point-bar deposits in both fields and from laterally equivalent crevasse-splay deposits at Lonetree field. Secondary porosity has created a common reservoir in point-bar and crevasse-splay depositsmore » at Lonetree. Laterally equivalent deposits of major trunk streams, such as the Kassler sandstone, generally produce only over structural noses in the southern D-J basin. Overlying J-1 sandstones form northeast-trending marine bars in both field areas. Two echelon bars are present at Poncho but only the western one is productive. The eastern bar at Poncho and the single marine bar at Lonetree are wet.« less

Diagenesis of Terry Sandstone (Upper Cretaceous), Spindle Field, Colorado

ABSTRACT This study discusses the influence that sedimentology, diagenesis, and basin subsidence history have on porosity and permeability of the Upper Cretaceous Terry sandstone of Spindle field in the Denver basin. The Terry sandstone consists of lithic subarkoses and feldspathic litharenites deposited in a marine bar complex. The reservoir facies, which was deposited in a marine bar crestal position, is a cross-bedded sandstone with a mixture of primary and secondary macropores. Approximately half of the pore volume is microporosity associated with clay minerals and lithic fragments. The nonreservoir facies was deposited on the bar-flank margin as lower-energy, alternating layers of mud and sand, which were extensively bioturbated to produce muddy sands. Ductile deformation of lithic fragments is a major factor in porosity loss for the reservoir facies. Chlorite and calcite are the most important mineral cements. Authigenic quartz, dolomite, ferroan calcite, kaolinite, feldspar, pyrite, and tourmaline are present in minor to trace amounts. Precipitation of these minerals is believed to be related to the evolution of marine water. Secondary porosity formed by dissolution of pore-filling and replacement calcite and framework feldspars. Sandstones with pervasive calcite cement have no secondary porosity developed, which suggests some permeability may be needed to develop secondary porosity. Plagioclase feldspars become more albitized with increasing depth. A temperature-depth subsidence profile predicts a maximum temperature of only 85°C, at least 15° lower than in Tertiary rocks, which suggests that kinetic considerations may also be important to albitization. Vitrinite reflectance (0.65%) and interbedded shales with ordered mixed-layered illite/smectite with up to 50% expandability also support the lower temperature regime. Calculations suggest that seawater can supply sufficient sodium to account for albitization. Albitization proceeded more rapidly in permeable sandstone, where the plagioclase was in direct contact with the pore fluids. Plagioclase in relatively impervious sandstones eventually became albitized, probably through diffusion.

Trace Fossils Indicating Estuarine Deposystems for the Devonian-Mississippian Cloyd Conglomerate Member, Price Formation, Central Appalachians

The use of ichnologic data is crucial to the understanding of some paleoenvironmental regimes. The Cloyd Conglomerate Member of the Price Formation has been interpreted as entirely fluvial, partly fluvial and partly marine bar, and entirely marine bar. Trace fossils provide data to resolve these divergent opinions. The Cloyd preserves a thin sedimentary wedge composed of a lower, regressive nonmarine phase and an upper, transgressive marine phase bearing trace fossils and a restricted marine fauna. Massive, quartz-pebble conglomerates of the lower Cloyd were deposited in fluvio-distributaty systems during a regression near the Devonian-Mississippian boundaty. Regressive channel systems became estuaries during a subsequent transgression. Trace fossils in the upper Cloyd constitute a mixed Skolithos-Cruziana Ichnofacies of suspension-feeding sand dwellers and depositfeeding mud dwellers. Though not abundant, trace fossils occur most commonly in the uppermost sandstone beds of fining-upward, estuary pointbar deposits. A point-bar assemblage includes large, deep Skolithos and Cylindrichnus suspension feeders that inhabited bar slip faces. Deep burrowing behavior suggests that point-bar trace makers sought to moderate environmental extremes, e.g., salinity and temperature fluctuation and possible subaerial exposure of bar surfaces. The Teichichnus trace maker inhabited lower-energy, submerged sand flats on point-bar surfaces, possibly following channel abandonment. Tidal-flat facies composed of organic-rich silty shales with interbedded siltstones and sandstones overlie point-bar sandstones. Well-circulated tidal-flat facies are dominated by shallow-burrowing suspension feeders in sandstone interbeds. The trace assemblage includes Arenicolites, Diplocraterion, Monocraterion, Planolites, Rusophycus?, and shallow Skolithos. A backwater assemblage includes Chondrites and Planolites deposit feeders that inhabited organic-rich, black