Grainstone Shoals Research Articles

Sequence Stratigraphy, Sedimentology, and Reservoir Characteristics of the Middle Cretaceous Mishrif Formation, South Iraq

The Cenomanian–Early Turonian Mishrif Formation is a great contributor to oil production in Iraq. Integrating petrographic, mineralogical, and wireline logging data from 52 wells, this study provides an improved understanding of the sequence stratigraphy, depositional evolution, and reservoir characteristics of the Mishrif Formation in the Mesopotamian Basin, south Iraq. Five types of facies associations are classified: lagoon, shoal, rudist bioherm, shallow marine, and deep marine. Such a classification allows convenient differentiation and interpretation of wireline logs. A sequence stratigraphic framework including five third-order sequences (Mhf 1 to Mhf 5) for the Mishrif Formation is established mainly using wireline logging data of close-distance wells, with the aid of cores and thin sections. Two end-member depositional evolution stages are recognized, from clinoform-like progradational shoal complexes in Mhf 1 within a shallow marine environment, to tidal channels in Mhf 2–3 within a lagoon environment. For Mhf 4–5, abrupt changes in facies associations from north to south indicate the development of an intra-shelf basin where organic-rich mudstones directly overlie the shallow marine grainstone shoals and lagoonal wackestones. Reservoir characteristics and compartmentalization are directly controlled by the sequence stratigraphic framework. Sequence boundaries are featured by wackestones and mudstones overprinted by cementation; they are regionally correlatable and work as regional barriers. Shoal complexes in Mhf 1 and tidal channels in Mhf 2–3 are the main reservoir units. Mudstones and wackestones are intra-reservoir baffles and become more frequently developed towards the south, reflecting the increasing water depth towards south. The characterization of the tidal channels, clinoform-like shoals, and intrashelf basinal deposits in the current study could benefit later development of the Mishrif Formation.

Lithofacies, Deposition, and Clinoforms Characterization Using Detailed Core Data, Nuclear Magnetic Resonance Logs, and Modular Formation Dynamics Tests for Mishrif Formation Intervals in West Qurna/1 Oil Field, Iraq

Summary This study considered the complexity of Mishrif geology and its effect on fluid movement within and across Mishrif reservoir intervals. For this purpose, we analyzed the following items: the multiple interval communication with high permeability contrast, the geological setting of the upper Mishrif (mA) interval, the channel structure in the Lower Mishrif-Part 1 (mB1) interval, the thin layers in the upper part of Lower Mishrif-Part 2 (mB2U) of very high permeability, and the microporous interval of the lower part of Lower Mishrif-Part 2 (mB2L); none of them were well defined before this work. The bottom interval of Mishrif or Rumaila (mC) is predominantly microporous, and the best reservoir is at the top of intermediate quality. Two high-porosity layers are systematically found in the mC unit, which is casually referred to as “rabbit ears.” The mB2L contains grainstones in the far north of the West Qurna/1 oil field (WQ1). In the south of mB2L, some of the toe sets from the clinoforms in a distal depositional setting have developed into rather important vertical pressure baffles and barriers to vertical flow. The mB2U generally consists of grainstones with thin streaks of mudstone high flow layers (HFLs), and the rocks underneath are described generally as grainstone shoals. About 80% of stock tank oil originally in place (STOOIP) in mB2U exists in grainstones. There are no known microporous reservoirs in mB2U. The pressure difference across the boundaries between mB1 and mA can be positive or negative. At the base, mB1 channels are always in pressure communication with the mB2U below. The best flow from the mA comes from HFLs, which are found around faults. Reservoir quality within mA is generally best in the first section of the upper Mishrif (mAa), and the majority of STOOIP in mA exists in microporous rocks, while some 30% of STOOIP is contained in grainstones.

Facies and sequence stratigraphic architecture of the Mural Limestone (Albian), Arizona: Carbonate response to global and local factors and implications for reservoir characterization

AbstractThis study documents the detailed facies and sequence stratigraphic architecture of a multi‐cyclic patch‐reef and its associated ramp interior facies that formed during Oceanic Anoxic Event 1b in the Mural Limestone, Arizona, USA. Ramp interior facies are comprised of bedded wackestone/packstone, rudist build‐up and coral–algal patch‐reef facies located north of Bisbee, Arizona, at the Grassy Hill locality. The larger multi‐cyclic patch‐reef that developed coevallyca5 km to the south of Grassy Hill consists of a high‐angle windward margin with a narrowca70 m long reef frame containing vertically zonatedMicrosolena, Actinastrea, diverse branching coral and rudist assemblages, and an 870 m long low‐angle leeward margin comprised of reef debris rudstone and grainstone shoal facies. Similar reef geomorphology and orientation is documented across the Gulf of Mexico and reflects the shelf‐wide north to north‐east‐trending prevailing wind and current energies. Controls affecting reef formation and growth patterns include changes in accommodation space associated with low‐amplitude global sea‐level rise and regional thermotectonic subsidence, local accommodation space and nutrient fluctuations associated with the inner shelf depositional setting within a humid and siliciclastic‐rich environment. Four aggradational to retrogradational high‐frequency sequences are documented in Arizona: High‐frequency sequences 1 and 2 represent the first pulse of patch‐reef development in an overall second‐order marine transgression over the Sonora/Bisbee Shelf. These sequences correlate to δ13C signatures associated with Oceanic Anoxic Event 1b across the Gulf of Mexico and suggest that carbonate reefs persisted on the ramp interior during this time. High‐frequency sequences 3 and 4 record a second brief transgression and backstepping of reef facies followed by the final regression of shallow shelf carbonates that correlates to more robust patch‐reef development in Sonora, Mexico. The patch‐reef at Paul Spur is an excellent outcrop analogue for productive patch‐reefs in the Maverick Basin (Comanche Shelf) of Texas. Detailed facies mapping of this outcrop analogue shows that the greatest reservoir potential is contained within the backreef grainstone shoals where primary porosity of up to 15% is observed.

Controls on deposition of shallow marine carbonates and evaporites – lower Permian Gipshuken Formation, central Spitsbergen, Arctic Norway

AbstractThe interbedded shallow marine carbonates and evaporites of the 95 to 105 m thick lower Permian Vengeberget member in central Spitsbergen form a lowstand to possibly early transgressive wedge sandwiched between open marine highstand carbonates along the northern margin of Pangea. Deposition took place on a wide (much greater than 40 000 km2) evaporitic platform separated from the open marine environments to the east by an inferred shelf margin barrier. Based on detailed description of facies in unique mountain‐scale outcrops, five CaSO4 and nine carbonate facies have been identified, all reflecting deposition in shallow evaporitic environments. The gypsum beds represent subaqueous deposition in broad hypersaline lagoons whereas the carbonates were deposited in protected shallow marine to peritidal environments and ooid‐dominated grainstone shoals. Cyclicity is expressed both as lithological shifts from carbonate to CaSO4 deposition and in the stacking of facies during CaSO4 and carbonate deposition. The number of carbonate–evaporite cycles varies from 12 to 28, but the overall thickness of the succession varies only slightly as a result of variations in cycle thickness, from 0·5 to 13·0 m. Two basic types of carbonate units can be recognized and reflect a combination of allocyclic and autocyclic controls. In addition, differential subsidence along older north–south orientated lineaments added to the complexity of cyclicity. The study provides insight into the complexity of the controls on deposition, architecture and dynamics of large‐scale mixed carbonate–evaporite successions formed in the shallow inner part of an epeiric sea or platform.

Co‐existence of skeletal and ooid shoals as a result of antecedent topography—Cat cay shoal complex, Bahamas

AbstractHigh‐resolution seismic data reveal an unexpected Pleistocene topography underneath the Cat Cay shoal complex along the western margin of Great Bahama Bank, illustrating how Pleistocene topography focuses tidal flow to create different types of grainstone shoals. The 1–3 km wide and 35 km long shoal complex is composed of the Cat Cay ooid shoal that is a laterally continuous 8 m thick ooid shoal and a sequence of 300–600 m wide and less than 6 m thick skeletal‐dominated tidal deltas south of Ocean Cay. The skeletal tidal deltas overlie an irregular Pleistocene surface, while the Cat Cay ooid shoal is situated on a flat Pleistocene surface east of a Pleistocene rock ridge. This finding challenges the assumption that an antecedent high is needed for ooid shoal initiation. The base of the Cat Cay ooid shoal is an up to 4 m thick skeletal‐peloidal unit that is similar in composition to the skeletal tidal deltas south of Ocean Cay but their deposition was followed by an up to 4 m thick accumulation of ooids. The Pleistocene ridge west of the Cat Cay ooid shoal allowed accumulation of mud and peloids (the nucleus source), while to the south, muddy sediment was winnowed away and no ooids formed. The evolution of the two shoal types is ultimately the result of the presence and absence of antecedent topography adjacent to the shoal system, resulting in variations of mud accumulations and the formation of the nucleus in the ooid shoal. The coeval occurrence of ooid and skeletal shoals in the same complex implies that in the rock record, a vertical succession from oolitic to skeletal shoals does not indicate an environmental change such as climate or an anoxic event but rather a change in flow conditions created by antecedent topography.

Lithistid sponge-microbial reefs, Nevada, USA: Filling the late Cambrian ‘reef gap’

Cambrian–Ordovician sponge-microbial mounds in the Great Basin of the western USA reveal reef structure and composition immediately prior to the Great Ordovician Biodiversification Event (GOBE). Here we describe lithistid sponge-microbial reefs from the upper Cambrian (Furongian, Stage 10) strata of the Arrow Canyon Range, Nevada. The reefs are mound-like structures up to 1 to 2 m high and a few meters wide that consist of an unidentified thin-walled, bowl-shaped anthaspidellid sponge, columnar microstromatolite fabric, and the calcified microbe Angusticellularia. The reefs formed in low-energy, subtidal environments in which lime mud filled spongocoels and inter-reef spaces around undisturbed, in place, thin-walled sponges. The reefs colonized stable substrates provided by oolitic and bioclastic grainstone shoals. The mutually attached lithistid sponges form the main framework of the reefs. These thin-walled and bowl-shaped lithistids most likely were adapted to low-energy environments. Spaces beneath the overhanging sponge walls were filled by microbial carbonates. These include pendent micro-dendritic Angusticellularia attached to dermal sponge surfaces and upward-growing masses of microstromatolites. After death the lithistid spongocoels were mainly filled by micritic sediment that hosted soft-bodied burrowing organisms and keratose-like sponges. These lithistid sponge-microbial reefs, together with an earlier example of late Cambrian (Paibian) dendrolite-lithistid reefs in the same area, characterize skeletal-microbial reefs immediately prior to the GOBE.

Facies variability of a lower Aptian carbonate platform succession, Tirgan Formation, eastern Kopet Dagh Basin, northeast Iran

The Lower Cretaceous (Hauterivian-lower Aptian) Tirgan Formation is a carbonate platform succession in the Kopet Dagh Basin and is up to nearly 800 m thick. This study focuses on facies variability and depositional settings of the Tirgan in the eastern part of eastern Kopet Dagh where the formation constitutes the basal part of the lower Aptian deposits. The succession developed during the final stage of evolution of Cretaceous carbonate platform in the southern Eurasian margin and could demonstrate a synchronicity with comparable successions in other areas. In the study area, the Tirgan (48 m thick) overlies Berriasian–Barremian continental strata and underlies the lower Aptian deep marine deposits. Facies analysis of Tirgan determined several facies related to open marine, ooid grainstone shoal, protected lagoon, and lower intertidal facies belts of a carbonate system. Abundant ooids, calcareous green algae, mud size carbonate, lack of evaporite evidence, and the absence of basinal deposits record deposition in warm water shallow marine settings of a carbonate ramp under a relatively humid climate. The Tirgan encompasses a single third-order depositional sequence composed of transgressive and highstand systems tracts. The transgressive package consists of intertidal, lagoon, and open marine facies intercalated with storm and shale layers showing an overall retrogradational trend. However, the highstand package consists mainly of ooid grainstone facies and displays prograding clinoforms. The lowermost lower Aptian Tirgan sequence in the study area corresponds to the basal Aptian global third-order sea level cycle. Contact of Tirgan sequence with the overlying middle to late Aptian deep marine strata is abrupt, suggesting a drowning event, which was likely the result of early Aptian extraordinary global warming.

Facies and porosity origin of reservoirs: Case studies from the Cambrian Longwangmiao Formation of Sichuan Basin, China, and their implications on reservoir prediction

The dolostone of the Cambrian Longwangmiao Formation has been a significant gas exploration area in Sichuan Basin. In Gaoshiti-Moxi regions, a giant gas pool with thousands of billion cubic meters' reserve has been discovered. However, the origin of the reservoir and the distribution patterns are still disputed, eventually constraining the dolostone exploration of the Longwangmiao Formation. This paper focuses on the characteristics, origin, and distribution patterns of the dolostone reservoir in the Longwangmiao Formation based on: the outcrop geological survey, cores, thin-sections observation, reservoir geochemical characteristics study, and reservoir simulation experiments. As a result, two realizations were acquired: (1) The Cambrian Longwangmiao Formation could be divided into upper and lower part in Sichuan Basin. Based on the two parts of the Longwangmiao Formation, two lithofacies paleogeographic maps were generated. In addition, the carbonate slope sedimentary models were established. The grainstone shoals are mainly distributed in the shallow slope of the upper part in the Longwangmiao Formation. (2) The grainstone shoals are the developing basis of the dolostone reservoir in the Longwangmiao Formation. Moreover, the contemporaneous dissolution was a critical factor of grainstone shoal reservoir development in the Longwangmiao Formation. Controlled by the exposure surface, the dissolution vugs are not only extensively distributed, but also successively developed along the contemporaneous pore zones. Hence, the distribution patterns could be predicted. The geological understandings of the origin of dolostone reservoir in the Longwangmiao Formation show that the reservoir distributed in the areas of karstification in the Gaoshiti-Moxi regions, as well as the widespread grainstone shoals in the whole basin, are the potential exploration targets.

Evolution of the Arabian continental margin of the northern Dibba Zone, eastern United Arab Emirates and Oman

The southern margin of the Musandam Mountains (UAE and northern Oman) preserves a Late Permian to Late Cretaceous Neo-Tethyan platform margin sequence that provides important insights into the switch from deposition in shallow shelf to deeper-water margin slope environments. New biostratigraphic dating resolves ambiguities in previous works and better constrains the timing of the stages in the development of the south Musandam platform margin and their link to the wider Oman tectonic framework. The accumulation of Late Permian to Late Triassic mainly dolomitised shelf carbonates along this sector of the nascent Neo-Tethyan margin was ended by a phase of margin-edge collapse linked to the onset of a major phase of Neo-Tethyan rifting and extension, and deposition of a fore-reef talus assemblage. This is newly dated as also being Late Triassic (Norian-Rhaetian) and termed the Kharas Formation. Following non-deposition, or possible erosion of Early Jurassic deposits, differential subsidence in the Middle Jurassic led to deposition of on-lapping deeper-shelf/slope facies peri-platform lime mudstones, fine grained turbidites and minor, channelised coarser-grained graded conglomerates and packstones (Sumeini Group) along the southern side of the Musandam platform. In the west, oolitic peloidal packstone and grainstone shoals were deposited, here termed the Jarief Formation. Their Bathonian-Callovian age is coeval with the regional development of oolitic facies on the carbonate platform, linked to a mid-Jurassic sea-level highstand, and their redeposition into the adjacent Hawasina Basin as turbidites of the Guwayza Formation of the Hamrat Duru Group. Uplift and destabilisation of the margin of the carbonate platform throughout Oman in the Late Jurassic during a mild compressional regime cut off the coarser sediment supply to the Jarief Formation, which was superseded by deposition of Sumeini Group slope-facies lime mudstones with numerous intraformational truncation surfaces. Slope facies sedimentation then persisted along the length of the southern edge of the Musandam platform as the margin relaxed and subsided until the end of the Early Cretaceous. Uplift of the margin edge, related to changing regional compressive stresses as seafloor spreading increased in the South Atlantic and intra-oceanic subduction in Neo-Tethys initiated creation of the Semail Ophiolite, led first to collapse along the shelf edge, generating the thick and highly channelised post Aptian Ausaq Formation mega-breccias. This marked the end of the passive margin as a thick, Late Cretaceous predominantly shale and chert succession was deposited as the margin subsided with continent-ward migration of the foredeep that developed ahead of final emplacement of the Neo-Tethyan Hawasina Nappes and Semail Ophiolite during the Santonian.

Characterization of the Devonian Kharyaga carbonate platform (Russia): Integrated and multiscale approach

ABSTRACT The Devonian carbonate platform (Obj-2) of the Kharyaga field (Russia) has been operated by Total since 1999 and produces 42° API oil, rich in H2S. A multidisciplinary (sedimentological, structural, stratigraphic, and geomechanical modeling) and multiscale study was made of the field, integrating seismic, micro-resistivity log, and core data to characterize the reservoir. The Kharyaga carbonate platform is characterized by complex relationships among sea-level variations, subsidence, and sedimentation rates. The field is divided into a platform edge (barrier zone) in the southern area, and back barrier and lagoon environments to the north, each characterized by different facies distribution and dynamic behavior. The platform shelf and the reef flat show dome-shaped thrombolites; the back barrier is characterized by grainstone shoals associated with columnar stromatolites, whereas the lagoon comprises peloidal wackestones and packstones associated with laminated fenestral stromatolites. Significant and prolonged subsidence, in both the lagoon and the platform edge, is associated with partial emergence of the platform reef, as shown by karst development. Seismic interpretation highlighted both underlying north-south structures and syndepositional east-west collapse faults along the southern barrier edge. Two-dimensional and three-dimensional sedimentological modeling of the Obj-2, calibrated by well data and core analyses, allowed the simulation of alternative scenarios to explain the evolution of the carbonate platform during the Devonian and suggested the observed facies distribution is compatible with the presence of a fixed outer platform edge represented by east-west faults. The carbonate platform of the Kharyaga field exhibits complex behavior; hydrocarbon (HC) accumulation and flow is controlled by the interplay of primary porosity, karst distribution, and structural discontinuities (faults and fractures). Besides the role of the karstification, this paper mainly focuses on the role played by tectonic structures on the evolution of the carbonate platform and on the HC flow. From drill stem test data, east-west faults and karsts are considered as major permeability heterogeneities, but interference tests showed more complex fluid pathways at the smaller scale. Microresistivity images show dominant northeast-southwest and northwest-southeast fracture sets in the central barrier, forming an interconnected flow network with the seismic faults. Geomechanical modeling suggests that these fractures are subseismic structures linked to inherited, deep-seated faults active at the time of fracturing, in addition to the listric reservoir faults.

Calcified microbial reefs in Cambrian Series 2, North China Platform: Implications for the evolution of Cambrian calcified microbes

This study focuses on the microbial reefs of the Zhushadong Formation (Cambrian Series 2) in Shandong Province, China in order to understand the evolution of calcified microbes in the North China Platform during the Cambrian Series 2 and 3. The microbial reefs occur in a thin unit, ca. 3m thick, over an area of 1km2. They consist of three types of thrombolite based on their mesostructures: rimmed, grainstone-patch, and dendritic. The thrombolites mainly occur in various coarse-grained carbonate facies, including crudely stratified oolitic grainstone, stromatolitic grainstone, and disorganized limestone conglomerate. Calcified microbes in the thrombolites include Epiphyton, Kordephyton, a tubiform microbe, Bija, Tarthinia, Renalcis, Amgaina, and Razumovskia. The Zhushadong thrombolites were formed within a grainstone shoal, and experienced repeated burial and exposure. The rimmed thrombolite and grainstone-patch thrombolite experienced abundant input of carbonate grains (forming grainstone patches). In contrast, the dendritic thrombolite formed solely by calcification of microbes that mainly include Epiphyton, Tarthinia, and the tubiform microbe. The outer crusts of the rimmed thrombolite were formed by Amgaina, under high energy conditions.The diverse calcified microbes of the Zhushadong Formation form the earliest assemblage of their type in the North China Platform. Their descendants, mostly Epiphyton, subsequently thrived, forming a ca. 180m thick microbialite–oolite-dominated succession during the Cambrian Series 3 (Zhangxia Formation). Although the reefs in the Zhushadong Formation are much smaller than those of the overlying Zhangxia Formation, their calcified microbes are more diverse. This most likely reflects changes in depositional environments (e.g., abundant siliciclastic input and tidal effects vs. those of a stable carbonate platform), and/or global changes within reef environments (e.g., end-Cambrian Series 2 extinction of archaeocyaths and calcified microbes). A decrease in diversity of calcified microbes in the North China Platform, where archaeocyaths were absent, may help to account for evolutionary trends in calcified microbes that occurred independently of archaeocyath influence.

Mixed carbonate-clastic facies in the Eocene Kalatar Formation of the southwest Tarim Basin (NW China): tectonic and climatic controls

During the Eocene, the southwest Tarim Basin was a large epicontinental sea as a part of the Turun Sea in the eastern Paratethys. The marine succession of the Kalatar Formation is exposed along the piedmonts of the southern Tian Shan and western Kunlun Shan, and can be subdivided into three members. On the southern side of the Tarim Basin, a non-barrier shore-to-shelf, siliciclastic-dominated system, locally with carbonates and oyster biostromes, developed along the western Kunlun Shan, where river input led to a seaward arrangement of alluvial fan, foreshore, shoreface, and offshore facies. In contrast, on the northern side of the Tarim Basin, a carbonate ramp system formed along the southern Tian Shan, including shallow bioclastic-oolitic grainstone shoals, oyster beds, and red algal rudstones along with a number of evaporite units, which formed under hypersaline tidal-flat and lagoonal conditions. The basin center was a largely moderate deep and low-energy embayment facies. Deposition of the mixed carbonate-siliciclastic Kalatar Formation was controlled by tectonic activity, in particular the development of the Himalayan orogeny with the closure of Paratethys sea-level fluctuations, and arid-humid climate variations.

Geometry and origin of dolomudstone reservoirs: Pekisko Formation (Lower Carboniferous), western Canada

Abstract Dolomudstones of the Pekisko Formation in western Canada form small but important oil and gas reservoirs. The reservoirs are irregularly shaped bodies 1 km or so wide and commonly 5–8 m thick. Porosity development within the dolomudstones is a complex function of sedimentation, early facies-selective dolomitization and later telogenetic leaching of calcareous components. The carbonate sediment precursor of the dolomudstone, interpreted from relict textures preserved in chert nodules, was a microwackestone with abundant silt-sized skeletal fragments. Dolomudstone reservoirs are comprised of dolomudstones, calcareous dolomudstones, and subordinate interbedded dolowackestones and dolograinstones. Some dolomudstone reservoirs are contained entirely within grainstones. Others are capped by tight fenestral lime mudstone that has been dolomitized locally. Dolomitization has been most intense within the centres of these reservoirs, and dolomudstones grade laterally into calcareous dolomudstones. The association of facies indicates that microwackestones were deposited in subtidal intershoal and lagoonal environments on an inner ramp. Grainstone shoals provided a broad barrier that absorbed wave energy seaward of the lagoon. Fenestral lime mudstones accumulated in peritidal environments in restricted areas of the inner ramp, landward of the lagoon. Dolomitization is interpreted to have been early and selective to the microwackestone facies because it retained permeability or was reactive during early burial. Dolomitizing fluids were most probably derived from overlying formations and made their way downwards through spatially separated conduits. The Pekisko Formation was exposed and sculptured at several Jurassic-Early Cretaceous unconformities. During these times, sandstones and shales were deposited in solution cavities developed within the dolomudstones. Concomitant leaching of calcite increased porosity of the dolomudstone reservoirs.

Integrated seismic analysis of carbonate reservoirs: From the framework to the volume attributes

The recognition of the significance of seismic geometries and the depositional sequence concepts derived from them have revolutionized carbonate stratigraphy. New insight into carbonate platform evolution has been gained from outcrops by describing subseismic geometric and facies relationships within a larger seismic-scale chronostratigraphic framework. Integration of the outcrop and seismic and knowledge of the subseismic facies distributions has improved seismic stratigraphic prediction. Recently, seismic analysis of carbonate strata has focused on the generation of more accurate and robust stratigraphic frameworks and large-scale carbonate platform architecture. Delineating the depositional sequence framework provides a predictive way to map reservoir (e.g., grainstone shoals, reefal rudstones and floatstones, etc.), source, and seal lithofacies, and to qualitatively delineate the early diagenetic history of a platform (e.g., subaerial exposure at sequence boundaries). Pore systems are complex and reco...

GEOLOGY AND HYDROCARBON POTENTIAL OF THE SOUTHERN GEORGINA BASIN, AUSTRALIA

The Georgina Basin is an intracratonic basin on the central-northern Australian craton. Its southern portion includes a highly prospective Middle Cambrian petroleum system which remains largely unexplored. A plethora of stratigraphic names plagued previous exploration but the lithostratigraphy has now been rationalised using previously unpublished electric-log correlations and seismic and core data.Neoproterozoic and Lower Palaeozoic sedimentary rocks of the southern portion of the basin cover an area of 100,000 km2 and thicken into two main depocentres, the Toko and Dulcie Synclines. In and between these depocentres, a Middle Cambrian carbonate succession comprising Thorntonia Limestone and Arthur Creek Formation provides a prospective reservoir-source/seal couplet extending over 80,000 km2. The lower Arthur Creek Formation includes world class microbial source rocks recording total organic carbon (TOC) values of up to 16% and hydrocarbon yields up to 50 kg/tonne. This blanket source/seal unconformably overlies sheetlike, platform dolostone of the Thorntonia Limestone which provides the prime target reservoir. Intra- Arthur Creek high-permeability grainstone shoals are important secondary targets.In the Toko Syncline, Middle Cambrian source rocks entered the oil window during the Ordovician, corresponding to major sediment loading at this time. The gas window was reached prior to structuring associated with the Middle Devonian-Early Carboniferous Alice Springs Orogeny, and source rocks today lie in the dry gas window. In contrast, high-temperature basement granites have resulted in overmaturity of the Arthur Creek Formation in the Dulcie Syncline area. On platform areas adjacent to both these depocentres source rocks reached peak oil generation shortly after the Alice Springs Orogeny; numerous structural leads have been identified in these areas. In addition, an important stratigraphic play occurs in the Late Cambrian Arrinthrunga Formation (Hagen Member) on the southwestern margin of the basin. Key elements of the play are the pinchout of porous oil-stained, vuggy dolostone onto basement where top seal is provided by massive anhydrite while underlying Arthur Creek Formation shale provides a potential source.

Abstract: Diagenetic Influence on Reservoir Development and Quality in the Smackover Updip Basement Ridge Play, Southwest Alabama 

ABSTRACT Smackover reservoirs in the updip ridge play of south Alabama are developed over low to high relief paleotopographic highs on the pre-Mesozoic basement surface. These reservoirs occur within shoal grainstones or microbial boundstone/bindstones that have been modified by intense diagenesis. Diagenetic patterns, which vary dramatically over short distances, are principally controlled by paleotopography and eustatic sea level fluctuations. Diagenesis occurred in both eogenetic and mesogenetic environments. Eogenetic processes include grain micritization, precipitation of aragonite or Mg-calcite cements, physical compaction, precipitation of low-Mg calcite cements, dolomitization, dissolution, and precipitation of dolomite and aragonite cements. Mesogenetic processes include chemical compaction, precipitation of ferroan calcite, quartz, and ferroan dolomite cements, dissolution, emplacement of hydrocarbons, and precipitation of anhydrite and baroque dolomite cements. The products of this complex diagenetic history are heterogeneous reservoirs which consist of one or some combination of interparticulate, shelter, and vuggy porosity with lesser amounts of moldic and intercrystalline porosity. While diagenesis had a significant influence on reservoir quality, it did not significantly alter the geographic distribution of reservoir-grade rock. The primary controls on reservoir development are depositional. Exploration strategies should seek to identify paleohighs with enough relief for the development of microbial reefs and/or grainstone shoals.

Evolution of Cupido and Coahuila carbonate platforms, Early Cretaceous, northeastern Mexico

The Cupido and Coahuila platforms of northeastern Mexico are part of the extensive carbonate platform system that rimmed the ancestral Gulf of Mexico during Barremian to Albian time. Exposures of Cupido and Coahuila lithofacies in several mountain ranges spanning an ~80 000 km 2 area reveal information about platform morphology and composition, paleoenvironmental relations, and the chronology of platform evolution. New biostratigraphic data, integrated with carbon and strontium isotope stratigraphy, significantly improve chronostratigraphic relations across the region. These data substantially change previous age assignments of several formations and force a revision of the longstanding stratigraphy in the region. The revised stratigraphy and enhanced time control, combined with regional facies associations, allow the construction of cross sections, isopach maps, and timeslice paleogeographic maps that collectively document platform morphology and evolution. The orientation of the Cupido (BarremianAptian) shelf margin was controlled by the emergent Coahuila basement block to the northwest. The south-facing margin is a highenergy grainstone shoal, whereas the margin facing the ancestral Gulf of Mexico to the east is a discontinuous rudist-coral reef. A broad shelf lagoon developed in the lee of the Cupido margin, where as much as 660 m of cyclic peritidal deposits accumulated. During middle to late Aptian time, a major phase of flooding forced a retrograde backstep of the Cupido platform, shifting the locus of shallow-marine sedimentation northwestward toward the Coahuila block. This diachronous flooding event records both the demise of the Cupido shelf and the consequent initiation of the Coahuila ramp. The backstepped Coahuila ramp (AptianAlbian) consisted of a shallow shoal margin separating an interior evaporitic lagoon from a low-energy, muddy deep ramp. More than 500 m of cyclic carbonates and evaporites accumulated in the evaporitic lagoon during early to middle Albian time. Restriction of the platform interior dissipated by middle to late Albian time with the deposition of peloidal, miliolid-rich packstones and grainstones of the Aurora Formation. The Coahuila platform was drowned during latest Albian to early Cenomanian time, and the deep-water laminites of the Cuesta del Cura Formation were deposited. This study fills in a substantial gap in the Cretaceous paleogeography of the eastern Gulf of Mexico coast, improving regional correlations with adjacent hydrocarbon-rich platforms. The enhanced temporal relations and chronology of events recorded in the Cupido and Coahuila platforms significantly improve global correlations with coeval, economically important platforms worldwide, perhaps con

Stratigraphic and Diagenetic Controls on the Distribution of Reservoir Properties in the Devonian Nisku Formation at Joffre Field, Alberta: ABSTRACT

The Frasnian Nisku Formation facies succession in Joffre Field represents an overall major shallowing-upward cycle. Superimposed on this are three megacycles (fourth-order cycles). Several parasequences (fifth-order cycles) are also present. Initiation of the Nisku depositional sequence occurred as below wave-base ramp buildups overlying basinal to fore-slope facies. A ubiquitous deepening event followed bioherm development, with deposition of relatively deep subtidal facies grading up into shallow subtidal facies. This second shoaling event culminated in progradation of peritidal lithologies over shallow subtidal grainstones and packstones. Final flooding of the peritidal facies, coupled with basinward restriction, resulted in the deposition of a subtidal evaporate facies with minor subaqueous siliciclastic influx. Differences in facies comprising this shelf-interior setting and the shelf-margin setting are evident. However, the major depositional cycles can be correlated between these two settings, indicating a basin-wide, eustatic control on Nisku deposition. At Joffre Field, favorable reservoir properties are confined to the reef boundstone facies and the shallow subtidal packstone/grainstone facies. Reservoir heterogeneity is complicated by depositional and diagenetic factors. Early pervasive dolomitization and late anhydrite plugging contribute to reservoir heterogeneity in the field. Variations in unit thicknesses and interrupted lateral continuity contribute significantly toward difficulties in reservoir development of the field.more » Core data based isopach maps and cross-sections indicate three main bioherms that correlate well with the distribution of porosity in the lower megacycle. Topography on these bioherms controlled the thickness and distribution of facies of the middle megacycle. For example, grainstone shoals of the middle megacycle were properly located over bioherms.« less

Pore Geometry: Control on Reservoir Properties, Walker Creek Field, Columbia and Lafayette Counties, Arkansas

Walker Creek field in southern Arkansas produces hydrocarbons from oolitic packstones and grainstones of the Jurassic Smackover Formation. The relationships between pore geometry and reservoir quality in these rocks were evaluated using petrographic methods and mercury injection capillary pressure analyses. Results indicate that reservoir quality is controlled by pore geometry, which, in turn, is determined by depositional and diagenetic processes. Reservoir rocks at Walker Creek were deposited as prograding grainstone shoals in a shallow-water, high-energy environment. Diagenetic processes, including early marine cementation, compaction, and deeper burial pressure solution and calcite cementation, modified the original pore system. Primary interparticle porosity is the dominant effective pore type and is most important in terms of reservoir performance. Secondary microporosity is also abundant, comprising a significant percentage of total porosity (locally up to 100%), but is generally ineffective. The rocks were subdivided into lithofacies based on their depositional and diagenetic origin. Within each lithofacies, reservoir, marginal reservoir, and nonreservoir rock types are identified based on their pore geometry and characteristic capillary pressure curve. The reservoir rock types are less cemented oolitic and oolitic-skeletal grainstones with steeply sloping capillary pressure curves and flat plateaus indicating relatively homogeneous pore sizes that are well interconnected by large pore throats. The marginal to nonreservoir rock types include mudstones, wackestones, muddy packstones, and cemented grainstones with generally small interparticle and moldic pores. Gently sloping capillary pressure curves with poorly defined or no plateaus indicate variable pore and pore-throat sizes. A close correlation is found among parts of the capillary pressure curve shape, such as slopes, plateaus, and displacement pressure, and aspects of pore geometry such as pore-throat size, pore-throat size distribution, and pore interconnectivity. In addition, capillary pressure analyses provide guidance in evaluating overall reservoir quality by rock type, as well as answer specific questions regarding net pay criteria as a function of height above free-water level.

No reff-rimmed margins to the Permian carbonate platforms of Thailand

In the central Thailand platform, marginal buildups (Early Permian) are formed mainly by fossiliferous grainstones with boundstones occurring only as small biostrome. Boundstones have a dominant skeletal element of encrusting Archaelithoporella and Tubiphytes with calcisponges and abundant syndepositional marine cements. This boundstone texture is similar to that of many other well documented Permian examples, such as El Capitan, U.S.A. and Trogkofel, Austria. In the Ratburi area of Peninsular Thailand, small bioherms (Middle Permian) are interbedded within grainstone shoals. Boundstone textures are matrix rich, with bryozoa, the main skeletal element and Tubiphytes, the main binding element. The Peninsular Thailand buildups formed on platform interior ridges and not in a shelf marginal position. These small biostromal and biohermal buildups in central and Peninsular Thailand did not form massive shelf-margin reef bodies as have been described from Permian platforms in western U.S.A. and southern China.