Continuous Sand Bodies Research Articles

Determination of Reservoir Quality in Field “D” in Central Niger Delta, Using Well Log Data

Well log data was used in this study to assess reservoir properties of field "D" in the southern area of the Niger Delta. For successful petrophysical evaluation, three hydrocarbon-bearing reservoirs (reservoirs A, B, and C) were identified and correlated. The following metrics were tested to determine reservoir properties: porosity, permeability, shale volume, fluid saturation, and net pay thickness. The calculated reservoir property values indicate high reservoir quality. Porosity readings in well OTIG 2 are almost the same, averaging 20%, but values in wells OTIG 7 and OTIG 9 vary from 14-20%. The reservoirs' average permeability was greater than 100md. However, in wells OTIG 2 and OTIG 9, values steadily decline with depth due to compaction caused by the overburden pressure of the underlying rock. Hydrocarbon saturation values in well OTIG 2 are almost the same, averaging 60%, but vary from 60-70% in well OTIG 7 as well as 48-55% in well OTIG 9. Water saturation values in well OTIG 2 are almost the same, averaging 40%, but range from 30-40% in well OTIG 7 and 45-52% in well OTIG 9. The average bulk volume water values in well OTIG 2 are almost the same, averaging 8%, but range from 6-8% in well OTIG 7 and 7-9% in well OTIG 9. There is some evidence that reservoirs A, B, and C in well OTIG 2 are one continuous sand body. This is due to the fact that their porosity, bulk volume water, hydrocarbon saturation, and water saturation values are all roughly the same, and their depth values are all quite similar. The bulk volume water values support the hypothesis that these formations are homogeneous and near irreducible water saturation. The reservoirs found in the field contain hydrocarbons.

Early post-rift confined turbidite systems in a supra-detachment basin: Implications for the early to middle Miocene basin evolution and hydrocarbon exploration of the Pannonian Basin

Understanding the interplay of sedimentation and deformation style is essential to develop an adequate model of the evolutionary history of an extensional sedimentary basin. Due to their volume and temporal extent, late post-rift deposits are often in the focus of research; however, early post-rift deposits are less studied. The aim of the study is to reconstruct the tectono-sedimentary evolution of a syn -rift to early post-rift succession in the hanging wall of a low-angle detachment zone and to recognize the effects of this deformation style on turbidity current pathways and depocenters. The small Őri sub-basin of the western part of the Miocene Pannonian basin is used as a study area where the oldest early post-rift turbidites of ~15–12.8 Ma occurred. These sediments were deposited in a supradetachment setting, where inherited and actively forming structures controlled the sediment pathways and accommodation of both the distally and locally sourced marine turbidites. Seismic data, well logs and core information were integrated to identify and analyze unconformities, structural elements, and turbidite deposits. Syn-rift, early post-rift and late post-rift system tracts were identified. The syn -rift/post-rift unconformity is a result of combined eustatic sea level drop and the change in deformation intensity. The main low-angle detachment fault, orthogonal hanging wall transfer faults and related folds together controlled the complex synformal shape of the basin, which was related to major corrugations of the footwall detachment. Four turbidite phases were distinguished based on upward increasing sand content, areal extent, reservoir quality, and degree of confinement as the turbidity currents reached the basin margins. (1) Small fans deflected by transfer faults developed, followed by (2) an axial turbidite system that reached an anticline and was deflected by it. (3) Lateral confinement by the anticline increased significantly and thick sandstone bodies with abrupt pinch-outs formed, while the influence of smaller faults diminished. (4) The axial turbidite systems covered the basin floor and their interaction with basin margins created continuous sand bodies of confined character. The sand fairways and types of confinement of a supra-detachment basin differ from high-angle normal fault systems. The exhumed metamorphic rocks provided the sediment, and the extension-parallel antiformal and synformal corrugations acted as lateral confinement. The gentle back-tilt of the detachment hanging wall block exerted a frontal confinement for the extensive turbidite system at the distal part of the basin, but the gentle NE-ward dip over the detachment still permitted long run-out distances. • Combination of eustasy and tectonics leads to syn-rift-early post-rift unconformity. • Corrugation fold of a detachment creates an intricate topography for sediment flows. • Deformation activity decreases during deposition of early post-rift turbidites. • Lateral, frontal confinement of turbidity currents controls reservoirs quality. • Differences between sand fairways in high- and low-angle extensional systems.

Accumulation and exploration of petroleum reservoirs in west slope of northern Songliao Basin, China

Abstract On the basis of the present situation of oil and gas exploration and geological research of the west slope in the northern Songliao Basin, the factors controlling reservoir formation, oil and gas migration and accumulation, have been re-examined from the aspects of structure, deposition and reservoir formation. The results show that: (1) The west slope is a gentle slope which overlaps to the west, and nose structure is developed near the hydrocarbon generation depression, which is in the dominant direction area of hydrocarbon migration. A series of NE structural belts are developed on the slope and are favorable places for oil and gas accumulation. (2) The west slope can be further divided into the upper slope and the lower slope, and there are many kinds of oil and gas reservoirs, including structural, structural-lithologic and lithologic ones. In the upper slope, the major oil layer is Sartu controlled by structure; in the lower slope, multi-layers are oil-bearing, and the oil reservoirs are mostly composite ones. (3) Faults, unconformity surfaces and continuous sand bodies are the main channels of oil and gas migration; structure, sand body and fault jointly control the oil and gas enrichment in the slope; and the matching relationship between micro-amplitude and sand body, small fault and sand body control the oil and gas accumulation. On the basis of the above research, fine identification and effectiveness evaluation technology of composite trap has been developed through extensive study. Combination traps were identified by multiple technologies, including fault classification, micro-amplitude structure identification, fine sedimentation research, and lithologic trap identification by waveform indication inversion; and then the configuration relationship between fault and sand body, structural amplitude and sand body were analyzed to set up the evaluation criteria of effective traps. According to the criteria, the traps were selected to enhance the exploration success rate.

The pre-Late Wisconsin stratigraphy of southern Simcoe County, Ontario: implications for ice sheet buildup, decay, and Great Lakes drainage evolution

Recent three-dimensional mapping investigations in southern Simcoe County, Ontario, allow refinement of the existing regional stratigraphic framework. Analysis of 25 continuously cored boreholes has revealed a complex but consistent sediment succession that provides a record of the last two glacial cycles (Marine Isotope Stages 1–6). Five stratigraphic units (SU1–SU5) comprise the pre-Late Wisconsin record. The stratigraphy is floored by a presumed Illinoian glacial complex consisting of a lower, coarse-grained till (SU1), locally overlain by stratified glaciolacustrine and glaciofluvial sediments (SU2), but more commonly capped by a stone-poor, fine-grained till (SU3) of the Georgian Bay lobe. A widespread subaerial unconformity developed on the upper surface of SU3 contains organic-bearing, nonglacial deposits (SU4) ranging between 54 800 ± 3000 years BP (considered beyond the limits of radiocarbon dating) and 37 450 ± 590 14C years BP. SU4 is abruptly overlain by a thick succession of rhythmically laminated lacustrine muds graded upwards into glaciolacustrine silts and clays interrupted by regionally continuous sand bodies (SU5). The succession is capped (and locally truncated) by Late Wisconsin Newmarket Till. The sedimentary record of southern Simcoe County is correlated with other well-studied reference sections in southern Ontario and contains information that informs reconstructions of former ice extents in the lower Great Lakes region following the Illinoian glaciation. Several sediment units host aquifers, but limited thickness and spatial extent, as well as issues with naturally occurring dissolved gases and solids, restrict their use for groundwater supply.

Characteristics and Origin of Tight Oil Accumulations in the Upper Triassic Yanchang Formation of the Ordos Basin, North‐Central China

AbstractThe Upper Triassic oil accumulations in the Ordos Basin is the most successful tight oil play in China, with average porosity values of less than 10% and permeability values below 1.0 mD. This study investigated the geological characteristics and origin of the tight oil accumulations in the Chang 6 member of the Upper Triassic Yanchang Formation in the Shanbei area based on over 50, 000 petrological, source‐rock analysis, well logging and production data. The tight oil accumulation of the Chang 6 member is distributed continuously in the basin slope and the centre of the basin. The oil‐water relationships are complex. Laumontite dissolution pores are the most important storage spaces, constituting 30%‐60% of total porosity and showing a strong positive relationship with oil production. The pore‐throat diameter is less than 1 µm, and the calculated critical height of the oil column is much larger than the tight sand thickness, suggesting that the buoyancy was probably of limited importance for oil migration. The pressure difference between the source rocks and sandstone reservoirs is inferred to have provided driving force for hydrocarbon migration. Two factors of source‐reservoir configuration and laumontite dissolution contributed to the formation of the Chang 6 tight oil accumulations. Intense hydrocarbon generation and continuous sand bodies close to the hydrocarbon kitchen are the foundation for the large‐scale oil distribution. Dissolution of feldspar‐laumontite during the process of organic matter evolution generated abundant secondary pores and improved the reservoir quality.

Optimization of Fracture Spacing in Western Sichuan Tight Gas Based on Fracture Interference

Western Sichuan Depression is a typical tight gas reservoir, the effective stimulation methods for such reservoir is staged fracturing in horizontal wells. But, scholars failed to give the method to set reasonable fracture spacing in tight gas. In this paper, Western Sichuan tight gas reservoir continuous sand body model is established by numerical simulation, through which the gas reservoir production performance in different facture spacing can be learned. Numerical pressure in the vicinity of fracture to describe the scale of fracture interference by the law of pressure drop super position. Then, net present value is used to optimize fracture spacing. The result show that 73 m is the optimal fracture spacing of II B continuous sand body in XinMa blcok .The research method is not only applicable to continuous sand body of tight gas reservoirs, but can be extended to non-continuous sand body. Furthermore it plays a constructive role in promoting the fracturing exploitation in tight gas.

Testing high resolution numerical models for analysis of contaminant storage and release from low permeability zones

It is now widely recognized that contaminant release from low permeability zones can sustain plumes long after primary sources are depleted, particularly for chlorinated solvents where regulatory limits are orders of magnitude below source concentrations. This has led to efforts to appropriately characterize sites and apply models for prediction incorporating these effects. A primary challenge is that diffusion processes are controlled by small-scale concentration gradients and capturing mass distribution in low permeability zones requires much higher resolution than commonly practiced. This paper explores validity of using numerical models (HydroGeoSphere, FEFLOW, MODFLOW/MT3DMS) in high resolution mode to simulate scenarios involving diffusion into and out of low permeability zones: 1) a laboratory tank study involving a continuous sand body with suspended clay layers which was ‘loaded’ with bromide and fluorescein (for visualization) tracers followed by clean water flushing, and 2) the two-layer analytical solution of Sale et al. (2008) involving a relatively simple scenario with an aquifer and underlying low permeability layer. All three models are shown to provide close agreement when adequate spatial and temporal discretization are applied to represent problem geometry, resolve flow fields and capture advective transport in the sands and diffusive transfer with low permeability layers and minimize numerical dispersion. The challenge for application at field sites then becomes appropriate site characterization to inform the models, capturing the style of the low permeability zone geometry and incorporating reasonable hydrogeologic parameters and estimates of source history, for scenario testing and more accurate prediction of plume response, leading to better site decision making.

Architecture of a forced regressive systems tract in the Turonian Ferron “Notom Delta”, southern Utah, U.S.A.

The large-scale stratigraphic architecture of forced regressive deposits has been documented in many previous studies. Bed-scale facies architectural analyses of these deposits, however, are still very limited. The Cretaceous Ferron “Notom Delta” in southern Utah, U.S.A. contains a 20 km dip-oriented exposure of a stepped, forced regressive systems tract. The main focus of this paper is to reconstruct the paleogeography and depositional history of the systems tract based on detailed stratigraphic and facies architectural analysis using 23 geological sections, photomosaics, and walking out of beds. Internally, the systems tract consists of 6 parasequences, 11f to 11a from the oldest to the youngest. During the progradation of parasequences 11f to 11b the paleoshorelines were wave-dominated, as indicated by the abundance of HCS and/or SCS beds, wave-ripple cross-laminated beds, and the occurrence of diverse and robust ichnological suites attributable to the Skolithos and Cruziana Ichnofacies. Progradation of the wave-dominated shorelines resulted in more homogeneous and laterally continuous sand bodies. From 11b to 11a, however, there is a distinct change in paleoshoreline regime from wave-dominated to tide-influenced as indicated by the common occurrence of tidal facies in 11a, including: (1) lenticular, wavy, and flaser bedding and bidirectional dipping cross strata; (2) reactivation surfaces, double-mud drapes, and ripple cross lamination with opposing dips at the toe of large dune-scale cross sets; (3) inclined heterolithic strata (IHS) and sigmoidal bedding with tidal rhythmites; and (4) cyclic vertical variation in facies and bed thickness and the common occurrence of sand-mud couplets. These tide-influenced facies show overall lower bioturbation intensity (BI 0-3). Progradation of the tide-influenced shoreline results in more heterolithic delta-front facies. Tidal and/or tidal-fluvial channels further dissect delta-front sandstones, forming more isolated sand bodies. Data from this study, as well as previous work, show that width and thickness of the forced regressive parasequences are small, typically less than 5 km and 20 m respectively. In subsurface studies, identifying and correlating such small-scale parasequences using sparse data involve significant uncertainties. A combination of the diagnostic features indicating forced regression and different data sets is essential to better constrain the geometry and architecture these small-scale bodies.

Structural and stratigraphic relationships of the Palaeocene mounds of the Utsira High

ABSTRACTThe Balder and Ringhorne Tertiary oilfields of the Utsira High are a cluster of prominent Palaeocene mounds, whose presence has confounded geologists since they were first observed on 2D seismic data back in the 1960s and 1970s. Until recently, the Balder Field geologic model consisted of distal, deep‐water Heimdal, Hermod and Balder Formation sandy‐debrite and turbidite sands, compensationally stacked along the flanks of the Utsira palaeo‐high, with intervening layers of hemipelagic shale. Remobilization of the sands by large‐scale fluidization accentuated the high‐relief mounds and sand injections linked reservoirs that were originally isolated. Reprocessed seismic data show strong primary reflectors that cannot be reconciled with this model; continuous sand bodies are observed to cross‐cut normal, biostratigraphically constrained sequence‐stratigraphic reflectors. The implication is that many, if not most, of the Hermod and Balder sands are not depositional, but were emplaced by injection. Furthermore, most of the Palaeocene mounds are associated with major ‘tears’ in the normally smooth Top Cretaceous chalk seismic reflector, and overlying chalk ‘rafts’. The chalk rafts were formerly thought to be detached glide‐blocks, shed from distant scarps to the east, but the improved data show that they are always positioned above matching depressions in the main Chalk, implying a local origin. Some of the rafts are immense, exceeding half a kilometre in width and millions of tonnes in mass. We believe the association of the chalk features with the mounds is more than a coincidence and suggest that they are genetically related. A number of driving mechanisms have been proposed for the formation of the Palaeocene mounds of the Utsira High, including normal post‐depositional compaction, fluidization by earthquakes, overpressuring due to gas migration and gravity‐sliding back into the Viking Graben. Our observations are, however, more consistent with the mounds resulting from episodic supra‐lithostatic pressure escape from beneath the Chalk.

Prediction of stratigraphic compartmentalization in marginal marine reservoirs

Abstract Marginal marine depositional systems exhibit stratigraphic reservoir compartmentalization potential at three hierarchical scales. At each of these scales, stratigraphic compartmentalization potential can be related to the dominant depositional processes and accommodation:coarse sediment supply ratio (A/S) that are acting at the time of deposition. All three orders of compartmentalization potential must be considered in order to define optimal field development plans and completion strategies. The lowest order of compartmentalization is usually at the inter-parasequence scale. The parasequence is represented by a conformable succession of strata separated by marine flooding surfaces and as such it generally defines the basic flow unit in marginal marine systems. In systems tracts associated with relatively high A/S ratios, for example late Lowstand, Transgressive and early Highstand (steeply rising shoreline trajectories), vertical compartmentalization potential is relatively high because of the enhanced preservation potential of flooding surface shales under these conditions. In systems tracts associated with relatively low A/S ratios, for example late Highstand, Falling-stage and early Lowstand (flat, slightly rising and falling shoreline trajectories), vertical compartmentalization potential of parasequences is reduced because the potential for erosion of flooding surface shales by overlying deposits is high and hence potential for vertical sand–sand contact between parasequences is enhanced. The second level of compartmentalization hierarchy is the inter sand-body scale. Individual sand bodies are defined within parasequences. The lateral connectivity of these sand bodies is a product of the dominant depositional processes active at the time of their deposition (wave, tidal, fluvial). Wave-dominated systems tend to produce more laterally continuous sand bodies, fluvial-dominated systems more laterally restricted sand bodies and tide-dominated systems both laterally continuous and laterally restricted sand bodies. Vertical compartmentalization potential of these reservoir sand bodies is related to A/S regime. In high A/S regimes, sand bodies are more likely to be disconnected or compartmentalized. In low A/S regimes, erosional amalgamation of sand bodies is more likely thereby leading to lower compartmentalization potential. The third order of potential stratigraphic compartmentalization is the intra sand-body scale. This scale is represented by intra sand-body heterogeneities such as dipping or horizontal shales, carbonaceous-rich beds or laminae, shale abandonment plugs of channels and carbonate concretions. In high A/S regimes the preservation potential of these heterogeneities is relatively high leading to an enhanced potential for intra sand-body compartmentalization. Lower A/S regimes result in a greater likelihood of lateral and vertical erosion of these heterogeneities leading to a higher potential for reservoir connectivity.

Subsurface Study of Lower Tuscaloosa Formation at Olive Field, Pike and Amite Counties, Mississippi: ABSTRACT

ABSTRACT Olive Field is located on the Amite-Pike County line in southwestern Mississippi. The field was discovered by Shell Western Exploration and Production Inc., and produces from a sandstone within the Lower Tuscaloosa Stringer Member; the trap is stratigraphic. The Stringer Member consists of alternating sandstones, siltstones, and shales, which have an average thickness of 230 ft (76 m) at Olive Field. The Stringer Member is a transgressive sequence deposited on eroded Washita-Fredericksburg sediments and is overlain by Middle Tuscaloosa marine shales. The coarse-to fine-grained sandstones are quartz-rich with a high percentage of sedimentary rock fragments. The majority of sandstones are shale arenites. However, a large number of rock fragments were badly altered; these fragments could not be identified with confidence; many may be of volcanic origin. The sandstones are cemented by quartz overgrowths, Fe-rich carbonate cement, and chlorite. Sandstones commonly have a high preserved interparticle porosity that is characteristic of Lower Tuscaloosa sandstones. This interparticle porosity is attributed to chlorite, which occurs as an isopachous cement that coats grains and reduced nucleation sites for quartz overgrowths. The Stringer Member was subdivided into five zones in this study based on lithology and electric log responses. The lower three zones were deposited in a fluvial environment. The sandstones were deposited as point bars and channel sands in a meandering system. Siltstones and shales represent overbank, flood basin, and channel fill deposits. The upper two zones consist of sediments deposited at or near sea level. Finely laminated shales and siltstones represent marsh and bay deposits. The topmost zone is a thin continuous sand body that represents a remnant beach deposited before major marine transgression.

Petroleum Geology of East Dykesville Field, Smackover C Sand,"Claiborne and Webster Parishes, Louisiana: ABSTRACT"

The discovery in 1980 of gas production in the Smackover C sand in the East Dykesville field of Claiborne and Webster Parishes, Louisiana, extended the productive limits of this reservoir 6 mi (10 km) south of the production in the Haynesville field. The development of East Dykesville field has revealed three productive fault blocks within an area 6 mi (10 km) by 3 mi (5 km). The Smackover C and B sand of East Dykesville are present 700 ft (213 m) above the Louann Salt as a portion of a more or less continuous sand body covering an area 9 mi (15 km) from east to west. This sand body extends southward from the Arkansas-Louisiana state line for more than 10 mi (16 km), and also produces at the Haynesville field. Production has been encountered in the C sand at East Dykesville from 10,912 ft (3326 m) subsea down to 11,605 ft (3536 m) subsea, an interval of 693 ft (211 m). The source of the sediments which constitute the Smackover C sand appears to be north of the sand body, as it thickens to more than 100 ft (31 m) in the Red Rock-Haynesville area and thins southward. Themore » sand also thins both to the east toward Haynesville and to the west toward Shongaloo. The C sand is 60 ft (18 m) thick in the north portion of East Dykesville field and thins to 20 ft (6 m) in the most southern wells. Isopach studies suggest a submarine-fan depositional environment on a stable shelf.« less

Depositional Processes in Deltaic Environment: ABSTRACT

Factors controlling depositional processes are analyzed for three of the world's major river delta systems: the Mississippi, Ganges-Brahmaputra, and Mekong. Despite variations in amount of factual data available it is apparent that these three river systems have developed different delta types which are of interest to the geologist attempting to compare ancient with modern deltaic environments. Four basic factors control and influence delta formations: (1) sediment load--both quantity and particle sizes transported by the river to its delta; (2) river regime--in particular its sediment-carrying ability and seasonal variations in that ability; (3) coastal processes--essentially the influence of waves, currents, and tides in the deltaic environment, and (4) structural behavior of the depositional site--progradation across a stable platform or subsidence contemporaneously with sediment deposition. Although the deltas which are considered here differ to some degree in most of the parameters involved, certain factors play dominant roles. The Mississippi delta area is characterized by rapid subsidence and its deposits are subject only to minor modification by coastal processes. Several overlapping deltas have formed during the Recent as sedimentation sites have shifted systematically during progradation. Rapid burial and minor modification by coastal processes preclude the development of laterally continuous sand bodies. The Ganges-Brahmaputra delta is tectonically controlled. River courses and sedimentation sites have shifted erratically in response to structural change. Coastal processes are dominated by high tides which have created an extensive tidal plain of predominantly fine-grained sediments. Laterally continuous sands are not typical of this delta. The Mekong has created a single delta system which has prograded across an essentially rigid platform. Contemporaneously, coastal processes (tides, waves, and currents) have reworked the delta front creating sand beaches which merge laterally to form sheet sands in the deltaic plain. End_of_Article - Last_Page 475------------