The Late Triassic to Early Jurassic Minjur Formation is an unconformity-bound, transgressive-regressive sequence composed of channelized fluvial sandbodies and coastal floodplain/playa mudrocks deposited along the northern margin of Gondwana. A comprehensive sedimentological, sequence stratigraphic and seismic geomorphological analysis of the Minjur Formation in Abu Dhabi has significantly enhanced our understanding of its depositional environments, the controls on reservoir distribution and its overall exploration potential.Reservoir distribution within the Minjur Formation has been interpreted through an integrated approach involving seismic geomorphology, core and wireline log analysis and regional stratigraphic correlations. In the southwestern onshore region of Abu Dhabi, sinuous, potentially marine-influenced axial channel networks indicate persistent sediment transport towards the Neo-Tethys Ocean to the north. Conversely, the eastern onshore region exhibits a broader drainage system composed of smaller, northeastward-flowing isolated and branching channels. These channels tend to narrow downstream, suggesting more intermittent flows that probably terminated inland within floodplain or playa settings.The deposition of the Minjur Formation was probably influenced by a combination of tectonic activity, localized halokinesis, eustatic sea-level changes and climatic variability. From an exploration perspective, the pronounced lithological contrast between the channelized sandbodies and surrounding floodplain mudrocks, along with the presence of bounding unconformities and syndepositional halokinesis, offers substantial potential for stratigraphic trapping throughout the region.

An integrated sedimentological and sequence stratigraphic analysis of the mixed carbonate and clastic Albian Mauddud Formation in the Bahrah and Sabiriyah fields provides an improved understanding of this important reservoir in north Kuwait. The reservoir comprises two broad stratigraphic units: the lower Mauddud represents the culmination of a long-term transgressive retreat of the underlying Burgan delta system, whereas the overlying upper Mauddud records a low-gradient carbonate shelf or ramp system. A highly layered reservoir architecture of carbonate and clastic facies suggests a combination of high-frequency relative sea level and/or climatic fluctuations. Sedimentological analysis and regional correlations reveal a complex mixed carbonate-clastic system in which there is a tripartite arrangement of proximal coarse grained deltaic, distal prodeltaic, and offshore to along-strike carbonate facies. These form correlatable reservoir units bound by flooding surfaces and/or significant influxes of clastic sediment. The distribution of clastic and carbonate facies produces a pronounced variation in reservoir properties, between the neighboring Bahrah and Sabiriyah fields of north Kuwait. Primary stratigraphic and depositional reservoir heterogeneities are controlled by carbonate facies transitions from poor quality, micritic lagoonal facies to high-quality, grain-rich shoals and localized, reworked rudistid build-ups. These carbonate facies are cyclically layered with deltaic sandstone and prodeltaic shales. In combination with secondary diagenetic (e.g., cementation and dissolution) and structural characteristics (e.g., faults and fractures), this facies variability exerts a control on reservoir quality and produces a complex mosaic of reservoir heterogeneity. Reservoir variability controls differences in production performance of the neighboring fields, and these have been accommodated by contrasting field development strategies.

The carbonate-dominated, mid-Cretaceous to early Cenozoic stratigraphy of north Kuwait exhibits seismic evidence for karstification at multiple stratigraphic levels. Seismic coherence and attribute data sculptured across mapped stratigraphic horizons, isochrons and cross-sections reveal a complex distribution of karstic geomorphology. Seismic facies have been rationalized into seismic facies associations (SFA) - SFA-1: small-scale dolines, sinkholes and polygonal channels, SFA-2: large-scale dissolution pipes and coalesced pipes, SFA-3: karst-modified faults and collapse zones, SFA-4: large fluvial channels and karst-related tributaries and SFA-5: massive undifferentiated karst. Karstification shows a strong link to the underlying structural framework, with intense karstification and partial collapse associated with transtensional/transpressional fault systems, and the common alignment of karst-related features along individual fault segments. Karstification is particularly focused across the top of the Campanian Hartha Formation, where it is manifested by small-scale dolines, sinkholes, and polygonal channels. Below, the Campanian to Santonian Mutriba Formation displays a complex array of large-scale dissolution pipes and coalesced pipes, which, together with karst-modified faults and collapse zones, penetrate several hundred meters into the underlying stratigraphy. Above the Hartha Formation, the top of the Maastrichtian Tayarat Formation is dissected by fluvial channels which drained the relative upland of the Dhabi Plateau situated in the south of the study area. Cross-sections showing local thickening of early Cenozoic Rus Formation stratigraphy suggest differential subsidence and superstratal sag linked to Paleocene collapse of the underlying karstified Cretaceous fault systems. Evidence for surface processes such as fluvial incisions suggest emergence and a dominance of epigene karstification in the latest Cretaceous (Hartha and Tayarat Formations). This suggests relative sea-level fall and emergence, probably linked to foreland deformation adjacent to the Zagros margin. A prominent structural control on karst distribution, together with seismically resolvable pipes that transcend entire carbonate formations for 100's of meters, also suggests that surface terrains were overprinted and enhanced in a burial environment. This was probably by hypogenic fluid, driven by large-scale basin circulation related to Late Cretaceous/early Cenozoic foreland deformation.

The Pliocene reservoirs of the North Coast Marine Area (block NCMA-1) comprise shallow marine shoreface to shelf sandstones (up to 30 m [98 ft] thick) and interbedded shelfal mudstones organized into stacked upward-coarsening parasequences (ca. 25-70 m [similar to 82-230 ft] thick). Production behavior is strongly conditioned by the interplay of subregional sand distribution and aquifer connectivity, and intrafield reservoir architecture complexity, commonly in the form of clinoforms.The M2 parasequence is a well-connected reservoir unit and forms part of a late highstand systems tract with a strongly progradational architecture. Large-scale reservoir connectivity can be demonstrated by pressure communication in producing fields up to 10 km (6 mi) apart. The widespread nature of the sand also links the reservoir to a significant aquifer that provides pressure support during field depletion. In contrast, clinoforms that dominate the M2 reservoir architecture at a variety of scales reduce lateral connectivity. Wells situated in individual sand bodies show significant pressure interference during production, whereas wells that straddle bounding clinoforms (possible parasequence boundaries), although in pressure communication, show more subtle pressure interference. In the Chaconia field, clinoforms that bound lobate sand bodies in the M2 reservoir have also apparently slowed the onset of water cut within the field by several years and therefore increased reserves recovery. Similarly, in the M6 reservoir of the Ixora field a significant bounding clinoform has partitioned the field through much of its production life, but production data are now suggesting breakdown and late-stage pressure support across the same clinoform. The M6 parasequences form part of a lower quality early highstand systems tract and are more discontinuous than their M2 counterpart due to well-developed bounding clinoforms. As such they are not characterized by large active aquifers and do not benefit from such prominent long-term pressure support. The M4 reservoir is an isolated sand-body complex that forms part of a high-frequency lowstand complex. It shows similar stratigraphic heterogeneity to that displayed in the M2, with variations in pressure decline exhibited within individual wells of the same reservoir unit. Its isolated geometry means it is not connected to a regional aquifer and is therefore undergoing volumetric decline with no water breakthrough.

ABSTRACTThe mid-Cretaceous Mauddud Formation is the main producing carbonate reservoir in the Raudhatain and Sabiriyah fields of northern Kuwait. Historical field information and results from waterflood pilots indicate that reservoir performance in these reservoirs is controlled by geological complexity at several scales. A detailed, integrated sedimentological and biostratigraphic investigation of the reservoirs, combined with dynamic reservoir data, have provided an understanding of Mauddud reservoir heterogeneity and of the principle controls on reservoir matrix behaviour. The largely carbonate Mauddud Formation overlies the Upper Burgan Member, a thick succession of fluvio-deltaic deposits, and consists of a diverse suite of carbonate facies deposited in low to high-energy, shallow-marine ramp settings. The basal part of the reservoir comprises mixed carbonate and siliciclastic sediments and reflects the establishment of a carbonate-dominated regime during waning supply of Burgan siliciclastic sediment. This system was eventually drowned and covered by the Wara Formation, a shaly offshore succession that is also the reservoir seal.Sedimentary facies associations and microfossil assemblages within the reservoir are organised in a broadly upward-shallowing succession constructed of several transgressive-regressive cycles, which are defined by prominent, widely-correlatable flooding surfaces. Each cycle exhibits a characteristic internal stacking pattern of minor depositional cycles. Field-wide mapping and interpretation of facies within each cycle reveals a SW to NE, proximal to distal, trend consistent with regional seismic and palaeogeographic interpretations. The high-energy, inner to mid-ramp carbonate succession in the lower portion of the Mauddud reservoir is punctuated by siliciclastic incursions. Abrupt lateral facies changes, thickness variations, and local intra-reservoir erosion surfaces in this section suggest that deposition was influenced by subtle syndepositional tectonism. The upper part of the reservoir, in contrast, lacks significant siliciclastic influence and is made up of widely-correlatable, lower-energy carbonate facies, although local subtle facies variations show that the Raudhatain-Sabiriyah structures continued as palaeohighs during deposition. The contrast in quality between grain-dominated facies at the crests of the two structures and less grainy facies along their flanks was accentuated by carbonate cementation in the water legs of the reservoirs, largely in the form of calcite concretions of variable abundance. Cementation is most pronounced in low-energy wackestone facies, particularly in proximity to flooding surfaces where nodules may be amalgamated to form laterally continuous, cemented layers which are commonly fractured. Another significant, but contrasting, diagenetic modification within the reservoir was the generation of secondary macroporosity through dissolution of aragonitic skeletal components in a shallow to intermediate burial environment.The stratigraphic evolution of the Mauddud reservoir, and its diagenetic overprint, in addition to post-depositional fracturing and faulting, created reservoir heterogeneities, which are critical to reservoir performance; one of the most significant of these is the relationship between horizontal and vertical permeability. Parasequences dominated by high-energy inner ramp grainstones, thin inner ramp rudist-bearing tempestites, and vuggy and fractured rudist floatstones and rudstones constitute thief zones that represent major challenges to reservoir management. In contrast, some cemented layers and flooding surfaces support pressure differentials of up to several hundred pounds/square inch (psi), thus complicating sweep and promoting reservoir compartmentalisation. The strong facies, diagenetic and stratigraphic controls on the distribution of thief zones and intra-reservoir baffles demonstrates how important it is to comprehensively understand reservoir sedimentology and stratigraphy when devising long-term development plans for reservoirs of this deceptively simple character.More recent 3-D seismic data, production surveillance, and horizontal development wells show faults and fractures to be important heterogeneities in both reservoirs. Due to the immaturity of the water flood in the Mauddud reservoirs, the impact of these features on field and well behaviour is as yet unclear, but it is anticipated that the impact of such features on well and field performance will become more pronounced during later development.

Within the Nile Delta gas province, reservoirs are dominated by Pliocene slope-channel systems, which are spectacularly imaged on high-quality three-dimensional seismic data. This article deals with the detailed seismic geomorphology of the Sequoia channel system, focusing on the geometry and distribution of its component sand bodies and the impact they have on reservoir heterogeneity. The Sequoia reservoir serves as a potential analog for similar but less well-imaged, deep-water slope systems.The reservoir consists of a succession of sandstones and mudstones organized into a composite upward-fining profile. Sand bodies include laterally amalgamated channels, sinuous channels, channels with frontal splays, and leveed channels and are interpreted to be the products of deep-water gravity-flow processes. Above a major basal incision surface, the reservoir is highly sand prone and made up of laterally amalgamated channels. The medial section of the reservoir is more aggradational and exhibits laterally isolated and sinuous channels. Within the upper part of the reservoir, channels are smaller, straighter, and built of individual channels with associated frontal splay elements and less common leveed channels. The main channel system is buried by a prograding slope succession that includes lobate sand-sheet elements. The stacking of facies within the Sequoia channel system implies a punctuated waning of sediment supply prior to eventual abandonment.The Sequoia channel is interpreted to be the late lowstand to transgressive infilling of a third-order early lowstand slope incision. The channel fill is overlain by a mudstone unit, which delineates a major correlatable hot gamma-ray event, and on seismic data, is a prominent downlap surface and therefore a possible maximum flooding surface.The Sequoia channel system shows evidence for synsedimentary faulting, including a large-scale downdip widening of the channel and small-scale channel diversions and intraslope ponding of flows.Understanding reservoir architecture in terms of sandbody geometries and connectivity is vital within Sequoia because the gas column occupies the most complex and heterogeneous upper part of the reservoir. Correspondingly, the basal sand-rich part of the reservoir will significantly influence aquifer behavior during production.

Detailed outcrop Study of Miocene platform carbonates along the southwestern margin of the Gulf of Suez rift, together with an extensive review of comparable systems elsewhere, has generated new tectono-sedimentary models for marine rift-basin carbonate systems. Both syntectonic and post-tectonic platforms are described for various extensional settings in rift basins. Syntectonic platforms are defined as systems deposited during periods associated with active faulting in rift basins. Post-tectonic platforms are defined as carbonate systems deposited after periods of active extensional faulting that develop over preexisting, fault-generated rift topography. Carbonate systems that occupy transfer zones are also defined, which display the effects of active tectonism and intervening phases of tectonic quiescence. These platform types produce recurring tectono-sedimentological signatures recognizable throughout the stratigraphic record. The footwall margins to syntectonic platforms generally develop through the tectonic modification of existing platforms and display features associated with relative sea-level fall such as emergence and, depending upon the prevailing climate, meteoric diagenesis. In addition, fault activity truncates platforms that have prograded across the fault line during preceding phases of tectonic quiescence. Hanging-wall dip-slope margins can be complex, inasmuch as they display the effects of hanging-wall subsidence and Lip-dip footwall uplift. Down-dip of the fault-block fulcrum, depositional sequences are more likely to be bounded by flooding surfaces and display retrogradational to backstepping depositional sequences, while up-dip of the fault-block fulcrum, sequences may form as a result of forced regression driven by relative sea-level fall. In rift-margin areas, clastic influx is likely to play a more important part in tectonically active hanging-wall dip-slope stratigraphies because of the rejuvenation of adjacent footwall highs. Post-tectonic footwall platforms develop on the uplifted highs of fault blocks during periods of relative tectonic quiescence. Such areas reflect favorable shallow-water sites for carbonate production, because commonly they are isolated from significant clastic drainage fairways. The geometry of. a footwall margin is highly variable, because it is inherited from the morphology of the faulted margin. Low relief footwall margins produce progradational platforms that can bridge the block-bounding fault zone. High-relief blocks produce smaller, aggradational platforms in which the platform margin corresponds to the position of the previous fault scarp. Hanging-wall dip-slope margins commonly have large progradational geometries and typically evolve from ramp-type to rimmed-shelf morphologies through time. Platforms are more likely to form on rotated fault blocks that dip toward the rift margin or upon intrabasinal blocks that are set well away from rift-margin clastic input. In areas of-low clastic flux, transfer-zone platforms occur with either faulted (hard linkage) or low-relief and progradational (soft linkage) margins. Such platforms commonly pass laterally into rift-parallel footwall margins to produce complex platform-margin geometries. Hydrocarbon reservoirs in platforms from these settings have a range of tectonic, sedimentary, and diagenetic controls, but the 3D models presented form templates that can be used as analogues for exploration and/or production from fault-block carbonate platforms. The active (syntectonic) and passive (post-tectonic) platform models presented can be used at scales varying from individual depositional sequences to large-scale rift-basin stratigraphies characterized by episodes of tectonic activity and intervening periods of tectonic quiescence.

This study combines outcrop with seismic data, and tectonostratigraphic modelling to characterise the stratigraphy of Tertiary fault-block carbonate platforms. Seismic sections from the Middle East and Southeast Asia indicate that carbonate platforms commonly develop on fault-blocks in the late syn-rift to post-rift stage of basin evolution, they are characterised by retrogradational or drowning morphologies with deep water clastics/evaporites sealing the platforms. A new tectono-stratigraphic modelling program simulates the development of carbonate platforms on domino-style fault blocks and predicts a distinctive tectonic control on the evolving syn-rift stratigraphy. Detailed field work from a three-dimensionally exposed Miocene syn-rift platform from the Gulf of Suez (Gebel Abu Shaar, Egypt) shows the existence of unconformity-bound depositional sequences which are predicted by the program and are interpreted to have formed in response to fault-block rotation. The sequences are characterised by synchronous hangingwall subsidence and footwall uplift and erosion. The seismic images, the modelling and the outcrop study provide different levels of information on fault-block platforms and their characteristic stratigraphy. The scale of the modelling is of particular value as it provides a link between the seismic-scale imaging and outcrop data. (C) 1998 Elsevier Science Ltd. All rights reserved.

Serial variation between Micraster populations from successive zones in the Upper Cretaceous Chalk of Europe is widely cited as evidence for evolution at the species level, whether changes between species are interpreted as gradual or punctuational. That these changes were adaptive and represent an improved functional efficiency with time is also now widely agreed, if not whether the changes were independent of environmental change or a response to it. Dead specimens of Micraster were commonly encrusted by a wide variety of small invertebrates, presumably because they provided islands of hard substrate on an otherwise soft, muddy sea floor. Less commonly, there is evidence that living specimens of Micraster were susceptible to predation by gastropods and other organisms, one of the natural selection pressures favouring adaptation to a burrowing mode of life.