Oolitic Dolostone Research Articles

Study on reservoir characteristics, pore-throat structure, and origin of tight oolitic reservoirs: A case study of Triassic Feixianguan Formation, NE Sichuan Basin, SW China

The origin and pore-throat structure of different lithofacies are key issues in exploration and development of tight oolitic reservoirs. Based on core and thin section observation, four types of lithofacies can be recognized in the Feixianguan Formation oolitic reservoir: 1) oolitic limestone with intergranular pores (Lithofacies A), 2) oolitic limestone with mold pores (Lithofacies B), 3) oolitic dolostone (Lithofacies C), and 4) silty crystalline dolomite and fine crystalline dolostone (Lithofacies D). The subsurface core samples from the different lithofacies were studied using mercury injection, 3D CT scanning, and nuclear magnetic resonance, indicating that Lithofacies D possesses the best pore-throat structure and reservoir connectivity. The pore-throat structure of Lithofacies C is very similar to that of D, but the heterogeneity is much stronger. By comparison, the pore-throat structure of Lithofacies A and B is relatively poor. Although the pore heterogeneity of Lithofacies B is weaker than that of Lithofacies A, there is no effective throat connection in Lithofacies B. On the basis of oolitic shoal deposition, the factors controlling the origin of different lithofacies are meteoric freshwater leaching and then dolomitization. Meteoric freshwater leaching dominates the origin of Lithofacies B but does not affect the origin of Lithofacies C. Lithofacies C is jointly controlled by seepage-reflux and hydrothermal dolomitization fluids. The origin of Lithofacies D is only controlled by seepage-reflux dolomitization. Some other diagenesis effects may also have an impact on petrophysical properties of different lithofacies, but they do not play a decisive role in the origin of different lithofacies.

Reservoir characteristics and genesis of the Lower Ordovician Tongzi Formation in central Sichuan Basin, China

Although the Ordovician petroleum exploration in the Sichuan Basin has been developed slowly for a long time, deepening the research of reservoir characteristics and genesis is still a robust way to accelerate it. This paper characterizes the Ordovician reservoirs in the central part of Sichuan Basin and analyzes their genesis based on cores, thin sections, well logs and drilling data. The results show the reservoirs concentrated in the Lower Ordovician Tongzi Formation, which consist mainly of sandy oolitic dolostones, bioclastic dolostones, fine crystalline dolostones, sandy dolomitized conglomerates, sandy dolostones and dolomite sandstone. The porosity types include interparticle pore, intraparticle pore, moldic pore, fracture and vug. Throats are mostly necked and flaky. The Tongzi Formation reservoirs mainly experienced three types of diageneses, including compaction, cementation and dissolution. The reservoir development was controlled by multiple factors of deposition, diagenesis and tectonics; the shoal facies is the basis, the penecontemporaneous dissolution and dolomitization are the main driving force, and the burial dissolution is the aid. The study can provide reference for deepening the Ordovician petroleum exploration and development in the Sichuan Basin.

Comparison of differential diagenesis of two oolites on the Lower Triassic platform margin, NE Sichuan Basin: Implications for the co-evolution of rock structure and porosity

Two oolites of the Lower Triassic Feixianguan Formation on the margins of the Kaijiang-Liangping (K-L) Trough in the NE Sichuan Basin were deposited in the upper parts of similar shallowing-upward sequences, and are capped by a subaerial exposure surface. However, they have contrasting diagenetic histories and reservoir qualities: the sour oolitic dolostone reservoirs on the eastern side of the K-L Trough have significantly higher porosities than the sweet oolitic limestone reservoirs on the western side. A petrologic and geochemical comparison of the two oolites was carried out so as to investigate the role of diagenetic processes on the co-evolution of oolite structure and pore network. On both sides of the K-L Trough, eogenetic meteoric digenesis led to an inversion of texture with ooid dissolution and interparticle cementation in the oolite layers close to the exposure surface. The layers relatively far from the exposure surface mostly underwent recrystallization in association with connate seawater, resulting in neomorphic fabrics in the ooid cortices and less interparticle calcite cement. During the subsequent dolomitization, the early-formed oolite structure were mostly inherited by the dolostones on the eastern side, with fabric-retentive textures in the layers close to the exposure surface and fabric-destructive textures in the layers far from the exposure surface. During deeper burial the oolitic limestones were subjected to extensive calcite cementation and porosity loss, whereas the dolostones generally retained more porosity and have better connectivity of the micropore network. The higher porosity of the dolostones is ascribed to lesser chemical compaction and associated cement precipitation, rather than the creation of new pores. Porosity enhancement is closely dependent on the absence of the calcite cement related to pressure solution. The heterogeneity of the dolostone fabrics and the porosity was further enhanced by the subsequent recrystallization and the dissolution-reprecipitation process induced by thermochemical sulfate reduction, during which small amounts of net porosity are calculated to have been created. The oolitic limestones are the best reservoirs close to the exposure surface, whereas the most porous dolostones with the highest degree of fabric destruction are commonly some distance from the exposure surface. These results provide some insights into the diagenetic controls on oolite structure and porosity, and have implications for predicting the porosity of deeply-buried oolitic reservoirs, so have global theoretical and practical significance.

Reservoir characteristics, pore structure, and main controlling factors of oolitic shoal reservoir in Feixianguan Formation: a case study from eastern Kaijiang-Liangping Trough

The understanding of pore structures and controlling factors of different oolitic shoal reservoir types in Lower Triassic Feixianguan Formation around Kaijiang-Liangping Trough has been a key issue restricting efficient exploration and development of such reservoirs. Based on thin section identification, scanning electron microscopy, mercury injection, three-dimensional CT scanning, cathodoluminescence analysis, stable isotope, and rare earth element analysis, the Feixianguan Formation oolitic shoal reservoirs are classified into three types: mold-oolitic dolostone type, residual-oolitic dolostone type, and sparry-oolitic limestone type. The residual-oolitic dolostone possesses the best pore structure and reservoir connectivity. By comparison, the pore structure and connectivity of mold-oolitic dolostone and sparry-oolitic limestone are relatively poor. Based on deposition of oolitic shoals, the main controlling factors for pore structure difference are indicated to be meteoric freshwater leaching and dolomitization. The pore structure of mold-oolitic dolostone reservoir is transformed near the exposed surfaces in the upper-middle portion of shallowing-upward meter-scale cyclic sequences. The mold pores are formed by meteoric freshwater leaching, and then transformed by the late dolomitization. The development of residual-oolitic dolostone is not strongly affected by meteoric freshwater leaching effects, while transformed by seepage-reflux dolomitization. Hydrothermal fluids are involved in the dolomitization process of the oolitic dolostones mentioned above. The emplacement of hydrocarbon preserves primary intergranular pore in sparry-oolitic limestone. Other diagenetic effects are less important for the pore structure of oolitic shoal reservoirs in the study area.

Distribution patterns and controlling factors for reservoir characteristic difference of oolitic shoals, Feixian’guan Formation, eastern Longgang area, SW China

In eastern Longgang area of northeastern Sichuan Basin, the Kaijiang-Liangping trough was deposited with and leveled up by Feixian’guan sediments, where oolitic shoals exhibit progradation and upward migration. These oolitic shoals are classified into oolitic dolostone shoals and oolitic limestone shoals. The dolostone shoals occur within a narrow belt along Changxingian platform margin where the thickness of one single layer can be large; however, the cumulative thickness is rather small. On the contrary, the limestone shoals spread widely along shifted platform margin and is characterized by small thickness of one single layer and large cumulative thickness. Diagenesis has been found to play an important role in oolitic reservoir properties. The origin of low porosity and permeability in this extensive oolitic limestone reservoirs was attributed to burial cementation, while diagenetic processes, such as, reflux and hydrothermal dolomitization, have been identified as the primary factors for generating high-grade oolitic dolostones. Feixian’guan reservoir is characterized by low matrix porosity (which usually ranges from 2 to 3%). Existence of fractures may improve overall porosity and permeability of these reservoirs. The direct influence of these fractures may be to connect originally isolated pores to form a better connected pore-fracture system. Additionally, the fractures may have facilitated the migration of the organic acids that are associated with oil and gas, to help further enhance the quality of the reservoir through corrosion.

Contrasting diagenetic evolution patterns of platform margin limestones and dolostones in the Lower Triassic Feixianguan Formation, Sichuan Basin, China

Abstract Deeply-buried carbonate-reservoirs from the Lower Triassic Feixianguan Formation in the Sichuan Basin of China host extensive natural gas resources. These reservoirs are predominantly found in oolitic shoals, with the reservoir quality of dolomitized zones being higher than that of undolomitized limestone counterparts. Here we present a combination of petrographic, isotopic, fluid inclusion, and quantitative porosity data in order to understand and predict the diagenetic processes that have impacted the reservoir quality of dolostones and limestones. The porosity of limestones has been reduced to ∼7.5% due to calcite cementation, whereas the porosity in oolitic dolostones is not cemented with calcite and typically has ∼23.5% porosity. Dolomitization and concurrent early-diagenetic gypsum growth played crucial roles on the development and preservation of high porosity in the oolitic dolostone, first by stabilizing the rock fabric to inhibit loss of porosity during burial, and secondly through the generation of new porosity by dissolution of carbonate and anhydrite. A negative shift of δ 18 O and salinity values ( 13 C values (∼−20‰ VPDB) and δ 18 O (∼−10‰ VPDB) together with elevated calcite precipitation temperatures (>110 °C). It is likely that TSR was responsible for the formation of enlarged dissolution vugs that increased porosity by ∼2% in dolostones due to: i) anhydrite dissolution, ii) production of significant amounts of water resulting in formation water undersaturated with respect to calcite and dolomite, iii) generation of H 2 S, and CO 2 , and the consequent reaction of H 2 S with the siderite (FeCO 3 ) component in calcite and dolomite. This study demonstrates the importance of diagenesis in the formation of deeply-buried, high-quality reservoirs in ooid-dominated grainstones influenced by the presence of evaporites. Our results should be useful for guiding future exploration and reservoir developments in similar paleogeographic and diagenetic settings.

Relationship between heterogeneity and seismic velocities of the Yudongzi Triassic oolitic reservoirs in the Erlangmiao area, northwest Sichuan Basin, China

In this paper, we use a range of parameters to quantitatively analyze the heterogeneity of oolitic reservoirs aiming to establish a predictive relationship between the reservoir heterogeneity and seismic velocities. The Yudongzi Oolitic shoals on the Triassic carbonate platform margin in the Erlangmiao area, NW Sichuan Basin, contain two facies elements: a stabilized sand flat and a mobile fringe. The stabilized sand flat consists mainly of oolitic limestones with moderately sorted grains of 0.1–6mm diameter that account for 40–65% of the rock components. The mobile fringe is characterized by residual oolitic dolostones in which the grains are extremely sorted with a diameter of 0.1–0.5mm and account for 70–80% of the rock components. These oolitic shoals occurred in the lower part of the Feixianguan Formation in each of the four sequence sets (i.e., Pss1–Pss4), and they were continuously progradational from Pss1 to Pss4. Diagenesis and reservoir properties of the oolitic shoals are closely related to their depositional environments, with the mobile fringe tending to contain potential reservoirs. Predictive relationships between the geological properties and seismic velocities of the oolitic reservoirs show that the content, mean diameter, mean radius ratio, and standard deviation of grains and dolomite content are linearly related to seismic velocities, while the porosity has a logistical correlation with seismic velocities. The seismic velocities decrease either with an increase of grain content or dolomite content, or with a decrease of mean grain diameter, mean radius ratio or standard deviation, and they decrease rapidly at first, and then slowly with an increase of porosity. With these correlations, impedance data from seismic exploration could be used to predict depositional environments, mineral composition, and porosity distribution of the oolitic reservoirs.

Structural characteristics of intra-platform shoal in the Leikoupo Formation in the transitional zone of the central and southern Sichuan Basin

Abstract Through comprehensive research on drilling, logging and core data of T 2 l 1 1 sub-member, which is the major pay of the Leikoupo Formation in the transitional zone of the central and southern Sichuan Basin, the structural characteristics, distribution regularity and its major controlling factors of intra-platform shoal are studied. The major rocks of intra-platform shoal in T 2 l 1 1 sub-member include oosparite, residual oolitic dolostone, sparrenite, residual doloarenite and sparry doloarenite, sparry bioclastic limestone and residual bioclastic dolostone, which constitute oolitic shoal, arene shoal and bioclastic shoal. The intra-platform shoal is characterized vertically by shallowing-upward sequences, and consists of three sedimentary units, namely shoal base, shoal core and shoal cap. Besides, three microfacies can be distinguished laterally, shoal core, shoal edge and intershoal sea. The variation of water depth and hydrodynamic conditions caused by sea level change control the characteristics and superposed pattern of shoal body. Meanwhile, the palaeogeomorphology with alternating uplifts and depressions defines the differential distribution and lateral combination pattern of intra-platform shoal body.

Remarks on the Permian-Triassic transition in Central and Eastern Lombardy (Southern Alps, Italy)

The main lithological and petrographical characteristics of the Permian-Lower Triassic Orobic and Brescian successions in central and eastern Lombardy are briefl y recorded, especially with regard to the units cropping out below and above the P-T boundary. The lower formation is represented by the Verrucano Lombardo, which consists of continental, fluvial red clastics, barren of fossils, generally Late Permian (Lopingian) in age, whereas the overlying Servino Formation, which is represented by well-bedded clastic and carbonate polychrome sediments, generally rich in fossils, pertains to the Early Triassic (Induan-Olenekian). The sequences of the two above-mentioned areas differ at least in part, as proof of their regional division, probably because of an inherited paleotopography and syntectonic activity. Taking into account the units bracketing the P-T boundary, which represents the real topic of this work, the Verrucano Lombardo of the Orobic Alps is paraconformably covered by the conglomerates and sandstones of the Prato Solaro Member in the lower part of the Servino Formation, cropping out extensively, although discontinuously, from the eastern side of Lake Como to the upper Scalve Valley in the Camonica region. The shape of some quartz rock fragments, derived from the Variscan crystalline basement and its Upper Carboniferous siliciclastic cover, has been interpreted as due to relatively coeval aeolian activity, and testifi es to an arid climatic “event” probably late Dienerian-early Smithian in age. In contrast, in the Brescia province, the onset of the Servino is made up of wave and current rippled, fi ne clastics, 1-2 m thick, and a typical horizon of oolitic dolostones (“Praso Limestone” Auct.), continuous from the lower Camonica Valley to the western Trentino. This unit could laterally correlate towards east, in the eastern South-Alpine segment, with the famous oolitic Tesero Member at the base of the Werfen Formation of the Dolomitic and Carnic Alps. In the Brescian Prealps, the above oolitic deposits crop out below some Claraia beds yielding forms common to those present in the Siusi Member of the Dolomites, generally attributed to late Griesbachian-early Dienerian times. Their age could be ascribed to a slightly older Griesbachian, i.e. to early Induan. Therefore, the P-T boundary in central and eastern Lombardy seems substantially located between the fi nal part of the Permian and the very base of the respective Triassic successions, temporally and spatially ranging in different ways and generally affected by non-depositional and perhaps tectonic processes. In our opinion, however, the duration of the gap, based on correlations with the well-documented stratigraphical studies recently carried out in the nearby Dolomitic area and other European regions, should be considered as slightly longer than previously recognized: the maximum gap could be estimated at about 3-4 Ma. As a consequence, we thus point out that the Servino Formation of the Brescian Alps rests, itself, paraconformably on the Verrucano Lombardo red beds, even if the P-T gap was probably less for correlation with the well-known Dolomites sections. At the end of the paper, for a more comprehensible understanding of the late- to post-Variscan geological scenario, is a tentative synthesis of the regional evolution.

Depositional environments and an apparent age for the Geci meta-limestones: Constraints on the geological history of northern Mozambique

Strongly contrasting rocks were juxtaposed during the long tectonometamorphic history of the Mozambique Orogenic Belt in northern Mozambique. The latest depositional event was expressed by accumulation of marine sediments, which were subsequently mildly metamorphosed and tectonically juxtaposed with granulite facies complexes. The low-grade metasedimentary rocks comprise the Geci group which was mapped, and studied petrographically, geochemically and isotopically in order to provide constraints on the depositional environments, age and latest history of the Mozambique Orogenic Belt. The group occurs as several large, tectonically dissected, intensively sheared, folded and mylonitised, SW–NE trending lenses within Unango Complex granulite rocks. In places, primary depositional features are well preserved. The dominant rocks are calcarenites, dolarenites, calcite matrix-supported and dolostone clast-supported carbonate breccias forming beds with erosional bases, normal and reverse graded bedding, and well-developed Bouma sequences. Dolomicritic, microbial and oolitic dolostone clasts were apparently derived from the margin of a shallow-water carbonate platform and redeposited by turbidity currents on a continental slope with calcareous sedimentation. The Geci meta-carbonate rocks have low SiO 2 and Al 2O 3 contents. MgO/CaO ratios fluctuate between 0.05 and 0.70 averaging 0.15 ± 0.18 (1 σ, n = 111). Acid-soluble constituents have moderate concentrations of Fe (777 ± 310 ppm), Mn (131 ± 85 ppm) and Sr (566 ± 145). Mn/Sr ratios are relatively low (0.26 ± 0.19), δ 13C and δ 18O values are invariably high, with only moderate scatter: +4.0 ± 0.6% V-PDB and 25.8 ± 0.5% V-SMOW, respectively. The least altered 87Sr/ 86Sr ratio of calcite is 0.70708, whereas dolomite is enriched in 87Sr with the lowest ratio of 0.70730. The least-altered 87Sr/ 86Sr and δ 13C ratios suggest an apparent depositional age of either 595–585 or 630–625 Ma. This provides a lower age limit for juxtaposition of the low-grade Geci group rocks and granulite facies rocks of the Unango Complex.

Facies analysis and petrophysical properties of the lithologies of the north gas field, Qatar

Petrographic analysis of selected core samples from Khuff (Permian-Early Triassic) and Hanifa (Lower Jurassic) Formations of Qatar indicates that their lithologies were deposited within a tidally influenced shallow-marine environment. The lithofacies of the Khuff and Hanifa Formations mainly consist of a basal sandstone unit overlain by carbonate grainstone, oolitic dolostone, bioclastic mudstone, and dolomicrite with anhydrite laths.

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

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

ORIGIN AND RESERVOIR ROCK CHARACTERISTICS OF DOLOSTONES IN THE EARLY TRIASSIC FEIXIANGUAN FORMATION, NE SICHUAN BASIN, CHINA: SIGNIFICANCE FOR FUTURE GAS EXPLORATION

Major discoveries of natural gas have recently been made in the oolitic dolostones of the Early Triassic Feixianguan Formation in NE Sichuan Province, Southern China. These dolostones were formed by three facies‐controlled dolomitization processes: (i) meteoric mixing zone dolomitization with dolomites having a relatively high degree of crystalline order (δ13C:−1.0 to 2.5%PDB; δ18O:−6.5 to −2.5%PDB); (ii) seepage‐reflux dolomitization associated with evaporative brines; the corresponding dolomite crystals are relatively ordered and were formed in tidal flat environments and platform‐margin oolitic shoals adjacent to lagoons; (iii) burial dolomitization (shallow to moderate burial depths, ca. 1,000 to 4,000m), whereby seawater‐derived brines were present in the host rock and the resultant water/rock reactions played a major role in dolomitization.The three dolomitization processes were controlled by the arid climate prevailing during the Early Triassic, and also by fourth‐order relative sea‐level changes, especially with respect to the reflux dolomitization. Burial dolomitization, which is of second‐order of importance for porosity development, was strongly dependant on the presence of sufficient original porosity to facilitate water‐rock reactions within the carbonates. The best reservoir rocks formed as oolitic banks and bars in the vicinity of evaporative lagoonal‐tidal complexes which experienced optimal conditions for dolomitization. Dolostones with a dolomite content of 80% to 90% form good vuggy reservoir rocks at the present day, indicating that the intensity of dolomitization influences the quality of reservoir rocks.According to our results, future gas exploration in the Feixianguan Formation dolostone reservoirs should focus on locating oolitic banks associated with evaporative lagoon and tidal flat complexes and delineating the best structural/lithological traps.

Subsurface arbuckle group (cambro-ordovician) in the Bowman #4 Well of the Wilburton field in the Arkoma Basin, Oklahoma: Depositional facies, diagenetic signatures, petrophysical aspects, and economic potential

Detailed petrographic and petrophysical analyses were performed on a core section of the Nicor #4 Bowman Well, Arkoma Basin, Oklahoma. The studied interval represents the Cambro-Ordovician Arbuckle Group. Petrographic analyses of core samples distinguish six lithofacies. They are as follows from the bottom to the top of the studied core: (1) Quartzose dolostone, (2) conglomeritic dolostone, (3) brecciated dolostone, (3a) crackle breccia, (3b) dissolution-collapse breccia, (4) oolitic dolostone, (5) stylolitic dolostone, and (6) stromatolitic dolostone. Diagenetic evolution has affected the petrophysical properties of these different lithofacies. The diagenetic history of the selected section records early to late dolomitization, stylolitization, dedolomitization (calcification), and silicification. Petrophysical analyses employing mercury porosimetric techniques determine three distinct petrofacies. These petrofacies are as follow: (1) low porosity and high recovery efficiency, (2) low porosity and low recovery efficiency, and (3) high porosity and high recovery efficiency. Based on petrophysical analyses, the best reservoir candidate is the crackle breccia petrofacies because it has intermediate porosity and high recovery efficiency values, plus this lithofacies is bounded by strata with low porosity and recovery efficiency values (impermeable layer). The Arbuckle Group carbonates were uplifted and subaerially exposed, which, led to the development of an unconformity surface. Consequently, karstification developed and modified various petrophysical properties of the core.

Associations de foraminifères benthiques dans les séquences de dépôt du Trias moyen?-supérieur de l'Atlas Tunisien central et méridional

In the Central and Southern Tunisian Atlasic Range, outcrops of the evaporite-dominating Mid?- andUpper Triassic series are restricted to tectonic thickening zones (NS and EW deep fault network), such as the Rhéouis and Sbiba domes, or at the base of Alpine overthrusts (North-South Axis, “Dorsale Tunisienne”). Poor in fossils, the Triassic of Tunisia nevertheless contains successive microfaunistic assemblages; they are Ladinian?-Carnian and Rhaetian in age, and occur within the oolitic carbonate wedges (TST of the 3 d order sequences) linked to the main sea level rises on this part of the South-Tethyan margin. The Ladinian?-Carnian foraminiferal assemblage with Aulotortus praegaschei-Triadodiscus eomesozoicus characterizes two distinct levels (F3b and F4) cropping out in the Rhéouis dome (North-South Axis) and situated within the “Calcaires noirs” and the “Gypses moyens” respectively correlated to the “Dolomies de Mekraneb” and the “Dolomies de Rehach” well known on the Saharian Platform. The Rhaetian assemblage F6 corresponds to the association of Hoyenella inconstans-Agathammina austroalpina-Gandinella falsofriedli, already identified in the Fkirine Fm. (“Dorsale Tunisienne”). In the studied area, it is present within oolitic dolostones included in the thick evaporites of the Rhéouis Fm., underlining the North-South Axis overthrust, just below the Liassic Nara Fm. dolomites. The F6 dolostones can be regarded as equivalent to the dolomites of the Saharian Zerzour Fm.

Lithostratigraphy and Hydrocarbon Potential of the Cambrian (Pre-Knox) Interval in the Conoco No. 1 Turner Well, Rough Creek Graben, Western Kentucky: ABSTRACT

In 1992 an important deep exploratory well was drilled in the Rough Creek Graben of western Kentucky. The Conoco No. 1 Turner well, in McLean County, reached a total depth of 14,202 feet in Precambrian granite. The objective of this well was to test potential gas reservoirs in Cambrian-age pre-Knox Group carbonates and synrift sandstones. The well encountered no commercial hydrocarbons, but provided new data on the evolution and hydrocarbon potential of the Rough Creek Graben. Encouraged by the Turner well, Conoco has drilled two additional wells in the graben. The Turner well penetrated over 4,000 feet of pre-Knox sedimentary rocks. The clastic rocks lack porosity in the Turner well (because of calcite and quartz cements), but may have reservoir potential in other parts of the basin. The oolitic dolostones contain dolomite cement, and also have no effective porosity. However, bitumen staining is present in the dolostones, indicating that oil moved through the interval. The Conoco Turner well has proven that substaintial thickness of synrift clastics and post-rift dolostones occur in the Rough Creek Graben, and that facies distribution is strongly tectonically controlled. Bitumen staining indicates that hydrocarbons were migrating in the basin, but prediction of effective porosity remains amore » problem in the search for Cambrian reservoirs in the Rough Creek Graben.« less

Petrological and isotopic implications of some contrasting Late Precambrian carbonates, NE Spitsbergen

ABSTRACTThe dominantly shallow‐marine Vendian succession of NE Spitsbergen contains distinctive types of carbonate rock. Limestones deposited before Vendian glaciation resemble those described from other Upper Proterozoic successions, being high in Sr and inferred to have been originally aragonitic, including the distinctive 5–10 Jim equant polygonal calcite of cemented shrinkage cracks. In contrast, manganoan stromatolitic limestones within marginal‐marine glacial‐outwash deposits, and consisting of micrite, microspar and fascicular‐optic calcite are interpreted as originally calcitic. The restriction of primary marine calcite to cold seawater is comparable with Recent and Permian carbonates, although the Precambrian example formed in a sea diluted with meltwater.There is good textural preservation of relatively 18O‐rich oolitic dolostones which were cemented in a supratidal environment by artesian fluids. Nevertheless, early diagenetic replacement is inferred, immediately prior to a glacial episode. Post‐glacial dolostones are either replacive marine, or evaporative lacustrine, but share rather more negative δ18O values, closer to the mean of Late Precambrian dolostones.The heaviest oxygen isotope values constrain seawater δ18O to no more negative than — 2 to — 4SMOW. The main reason for the pronounced oxygen isotopic depletion of most Late Precambrian carbonates is their initial metastable mineralogy. The possibility of determining palaeolatitudes of the enigmatic widespread Late Proterozoic glaciations by isotopic analysis of freshwater periglacial calcareous precipitates is raised. Significant carbon isotope variations reflect changes in depositional water chemistry: some of these could be global in extent.