Silty Dolomite Research Articles

Control of complex lithofacies on the shale oil potential in saline lacustrine basins of the Jimsar Sag, NW China: Coupling mechanisms and conceptual models

The heterogeneity of lithofacies is a pivotal factor influencing the shale oil potential. A comprehensive understanding of the intricate influence of rock complexity on hydrocarbon generation properties, microscopic pore-throat structure, and fluid occurrences is crucial for shale oil exploration and development. The Lucaogou Formation in the Jimsar Sag is a type of saline lacustrine shale oil, which was chosen as a research example in this paper. To unravel the causal linkage and confirm the coupling relationship between lithofacies, pores, and fluid mobility in the sedimentary sequence of saline lacustrine basins, a series of experiments were conducted. These included total organic carbon (TOC), X-ray diffraction (XRD), vitrinite reflectance (Ro), Rock-Eval pyrolysis, field emission-scanning electron microscopy (FE-SEM), low-field nuclear magnetic resonance (LF-NMR), mercury intrusion porosimetry (MIP), and low-temperature nitrogen adsorption (LTN2A). The results revealed significant differences in pores, oiliness, and fluid mobility among the four dominant lithofacies. Employing an innovative comprehensive evaluation method, high-carbon silty dolomite manifests significant potential for hydrocarbon generation, substantial storage capacity, effective oiliness and mobility, heightened brittle mineral content, and facile fracture production. Consequently, it was the most favorable lithofacies. Furthermore, TOC, pore size, and pore types emerged as key factors influencing the movable oil content of different lithofacies. More importantly, a detailed multiscale model illustrated potential linkages across the three different scales of lithofacies associations, pores, and fluid mobility. The study’s outcomes can offer enhanced guidance for accurately assessing shale oil resources, particularly in the identification of crucial “sweet spots”.

Lithological and diagenetic variation of mixed depositional units in the middle permian saline lacustrine deposition, Junggar Basin, NW China

Mixed depositional reservoirs are widely distributed in Junggar Basin, NW China. These reservoirs are featured by complex lithology, ultra-low permeability and extremely heterogeneous pore structures that markedly impact field development efficiency. This study illustrated the distribution of lithological combinations within a single sand bar in the Middle Permian Lucaogou Formation, Jimsar Sag, Junggar Basin and revealed the diagenetic control on the reservoir quality of a single sand bar based on thin sections, scanning electron microscopy (SEM), helium porosity and air permeability measurement, pressure-controlled mercury injection (PMI), rate-controlled mercury injection (RMI), X-ray computed tomography (CT), and collected and published data. The results show that the heterogeneity of microscopic pore structures within a single sandbar is controlled by the distribution of various lithological combinations and different levels of diagenetic alteration. The results show that there are mainly three types of lithology developed in a single sand bar. The reservoir quality of dolomitic siltstone reservoirs is the best, followed by silty dolomite reservoirs. The reservoirs quality of dolomicrite reservoirs is the worst. Three main lithological combinations can be identified within a single sand bar. Controlled by climate, lake level fluctuation and provenance, lithological combination A, characterized by blended mixing of dolomitic siltstone, silty dolomite and dolomicrite, is mainly developed in the middle of single sand bar. Lithological combination B, characterized by saltatory mixing of interbedded dolomitic siltstone and dolomicrite, and lithological combination A developed on the side of the sand bar near the shallow lake, while lithological combination C with blended mixing of interbedded dolomitic siltstone and silty dolomite developed on the side near provenance. Strong compaction is the main factor of the decrease of reservoir quality of sand bar reservoirs. Carbonate cementation promotes the densification of reservoirs. The irregular flaky clay minerals lead to the exponential decline of permeability, and dissolution is the main kind of diagenesis to improve reservoir quality. The dolomitic siltstone reservoirs in the middle of a single sand bar have the best reservoir quality because the overlying dolomicrite layers resist compaction, resulted in a certain amount of primary pores remained. Besides, the dolomitic siltstone reservoirs are far from the sand-mudstone interfaces, which leads to the low carbonate cement content. Furthermore, abundant dissolution pores in dolomitic siltstone reservoirs improve the reservoir quality. These research results are crucial to reservoir evaluation and development in similar mixed depositional tight reservoirs.

Rock physics characteristics and their control factors of carbonate in different sedimentary microfacies of the Yingshan Formation, Gucheng Area, Tarim Basin

Abstract Understanding the influence of geological characteristics on rock physics properties is crucial for accurately recognizing the relationship between rock physics variation and reservoir characteristics. Unlike the conventional rock species, the rock physics properties of the deep carbonate rocks in the third member of Yingshan Formation (Ying-III Member) in Gucheng area, Tarim Basin are relatively more complex. To address this problem, we investigated the rock physics characteristics and controlling factors of different sedimentary microfacies samples, combined with sedimentological analysis and rock physics experiments. The results show that the sedimentary environment affects the lithology and pore structure by controlling the properties of the primitive rock and early diagenesis. Dolomitized shoal microfacies and shoal top dolomitic flat microfacies primarily form crystalline dolomite and siliceous dolomite, with pores consisting of inter-crystalline pores, dissolution pores, and cracks. Inter-shoal dolomitic flat microfacies develops silty dolomite, with only a few inter-crystalline pores and cracks. Middle- to high-energy shoal microfacies and inter-shoal sea microfacies develop tight calcarenite and micritic limestone. Samples with similar mineral composition have relatively consistent density values and acoustic properties. Soft pores, such as micro-cracks, have a significant impact on the effective pressure and acoustic wave velocity, velocity and velocity ratio, and velocity and porosity relationships. The research shows a new approach for the rock physics characteristics of deep carbonate reservoirs under geological background constraints, as well as the rock physics basis for seismic prediction of the Ying-III Member reservoir.

Palaeoenvironment evolution and organic matter enrichment mechanisms of the Wufeng-Longmaxi shales of Yuanán block in western Hubei, middle Yangtze: Implications for shale gas accumulation potential

The deposition of black shale is influenced by several factors, including organic matter input, which is controlled by primary productivity; preservation, which is governed by redox conditions and deposition rates; and dilution, which is controlled by terrigenous input. However, determining the individual contribution of these factors to organic matter enrichment (OME) is complex due to significant variations in palaeodepositional conditions across different regions. In this study, we aimed to investigate the palaeoenvironmental conditions and OME mechanisms in the Wufeng-Longmaxi Formation in western Hubei by integrating data on total organic carbon (TOC) content, mineral compositions, and major and trace elements. We also examined the rapid variations in the palaeoenvironment, which led to the mass extinction in the Late Ordovician. Our findings indicate that dolomite siltstone and silty dolomite are present in the Guanyinqiao Formation (GYQ), which differs from other regions. The silica in the Wufeng-Longmaxi Formation has a mainly nonhydrothermal origin, while the upper Wufeng Formation (WF) has an entirely biogenic source. The organic-rich shales were primarily deposited in a warm and humid climate with low terrigenous input, low salinity, dysoxic conditions, and moderate to high palaeoproductivity due to moderate to strong upwelling. The GYQ serves as a transitional section from the WF to the Longmaxi Formation, which was characterized by a cold climate, oxic conditions, weak upwelling, and deposition in a strongly restricted setting, potentially leading to the mass extinction. Overall, OME was primarily dominated by preservation, with factors that influenced OME changing vertically. Palaeoproductivity, combined palaeoproductivity and palaeoredox conditions, and palaeoredox conditions were the main controlling factors in the WF, the Lower Longmaxi Formation (LLMX), and the Upper Longmaxi Formation (ULMX), respectively. Additionally, the Huangling basement, a submarine high, and the Xuefeng uplift's uplift and compression played important roles in OME. However, shale gas exploration prospects in the study area are limited due to the thinness of the organic-rich shales, oxic to dysoxic conditions, and relatively low palaeoproductivity.

Reservoir Characteristics and Controlling Factors of Oil Content in Hybrid Sedimentary Rocks of the Lucaogou Formation, Western Jimusar Sag, Junggar Basin

Hybrid sedimentary rocks (HSR) are a major reservoir type in unconventional oil exploration. The reservoir characteristics and controlling factors of the oil content of HSR are not clear, restricting the understanding of the storage mechanism and sweet spots within HSR. The Lucaogou Formation in the western Jimusar Sag is taken as a case study. Starting with the classification of the lithofacies system, differences in the microscopic pore structure, oil content and controlling factors of HSR reservoirs are revealed. The results show that seven lithofacies are recognized based on mineral composition, sedimentary structure, and organic matter characteristics, exhibiting rapid vertical and horizontal changes affected by the sedimentary environment. Layered mudstone lithofacies of the shallow lake mud and massive dolomitic mudstone lithofacies of dolomitic mud flats have the worst physical properties and oil content properties. However, they do have high organic matter contents and are the main source rocks of the Lucaogou Formation. The massive argillaceous siltstone and massive argillaceous dolomite lithofacies interbed frequently. Although their physical properties are moderate, “source-reservoir integrated” unconventional oil reservoirs can be formed, due to the adjacent to the source rock. Massive dolomitic siltstone, massive siltstone, and massive silty dolomite lithofacies are developed in the middle of the Lucaogou Formation, with the highest proportion of mesopores and macropores, which is indicative of good storage properties. When these lithofacies are filled with crude oil generated from source rocks at the top and bottom of the formation, “source storage adjacent” unconventional oil reservoirs be formed. The oil content is also controlled by the pore structure and specific surface area. The proportion of macropores and mesopores is positively correlated with oil content. The oil content of the samples is very low, when the specific surface area exceeds 2 m2/g. This manuscript provides a geological basis for evaluating and establishing reasonable interpretation models of HSR sweet spots.

Dolomitization of the Middle Ordovician Ma55 Sub-member of the Majiagou Formation and implications for hydrocarbon exploration in the northern Ordos Basin, NW China

The Middle Ordovician Majiagou Formation in the northern Ordos Basin consists of a sequence of subtidal and peritidal carbonates that are extensively dolomitized. The diagenetic evolution of the Ma55 Sub-member was investigated to determine the factors controlling reservoir quality in the northern Ordos Basin. Dolomite in the Ma55 Sub-member is classified into three main types on the basis of crystal size: dolomicrite (D1), micritic–silty and fine silty dolomite (D2), and silty (and coarser) dolomite (D3). The Dolomite D1 was produced by dolomitization in a low-salinity diagenetic environment and transformed by near-surface freshwater leaching during shallow burial. The Dolomite D2 developed as a result of post-penecontemporaneous seepage–reflux dolomitization and was affected by later burial dolomitization. The Dolomite D3 was produced during burial dolomitization. Although the original porosity of the Dolomite D1 was low, freshwater diagenesis increased the porosity significantly. As a result of the transformation of montmorillonite to illite, there was a deficiency in the Mg2+ necessary for Dolomite D2 formation, resulting in the nonuniform dolomitization of this type of dolomite. The intercrystalline pores of the Dolomite D2 are mostly filled with montmorillonite and illite, reducing its porosity and permeability and consequently making the Dolomite D2 a poor reservoir. The Dolomite D3 has a higher porosity and contains various types of dissolution and intercrystalline pores, generating a higher permeability. This dolomite is a potential reservoir in the study area and a possible target for oil and gas exploration. Overall, the results of the study suggest that the quality of a potential dolomite reservoir is strongly controlled by the diagenetic history of the host carbonates.

Paleoenvironments of Coniacian Age Outcrops in the Congo Coastal Sedimentary Basin

The study of the Turonian formations outcropping along the Congolese coast was undertaken in the order of to understand the sequential organization of the deposits as well as the evolution of the paleoenvironments. The study consisted of a detailed description of the lithofacies in the field and in the laboratory. This study shows that these formations are essentially made up of silty dolomites, formed in a littoral environment where the waters were favorable to the multiplication of planktonic foraminifers, Lamellibranchs and Brachiopods. The sedimentological characteristics indicate a sudden retreat from the shore, witnessed by the various regressive elementary sequences. These elementary sequences each begin with a layer of thanatocenosis which would be due to a sudden change of environment, consequence of an important arrival of detrital terrigenous sediments. This terrigenous detrital contribution disrupts each time the physicochemical characteristics of the environment (oxygen deficiency, absence of light, high turbidity). The fauna of these formations exhibits a coniacian affinity. The sequential evolution of the deposit shows a large ravine unconformity on the top of the mega sequence. The composition of the fauna and the sedimentological characteristics confirm the existence of a regressive sedimentation model of the Lower Coniacian age.

Geochemistry and provenance of siliciclastic rocks from the Mesoproterozoic Upper Vazante Sequence, Brazil: Insights on the evolution of the southwestern margin of the São Francisco Craton and the Columbia Supercontinent

Studies of provenance and tectonic settings of Proterozoic sedimentary basins on the margins of the São Francisco Craton (SFC) can enhance understanding of its evolution. The Mesoproterozoic Upper Vazante Sequence is made up of siliciclastic and carbonate rocks (mainly dolomites) that were deposited on the southwestern margin of the SFC between 1300 and 1100 Ma and then thrust eastward on top of the SFC during the Neoproterozoic Brasiliano Orogeny (650–550 Ma). The Serra do Garrote Formation, stratigraphically below the main carbonate sequence, is made up of phyllites, carbonaceous phyllites (>1% TOC), as well as minor meta-arenitic rocks. Serra do Poço Verde Formation meta-siliciclastic rocks, which occur interbedded with dolomitic carbonates, are made up of phyllites, carbonaceous phyllites, dolomitic phyllites, carbonaceous dolomitic phyllites, minor meta-arenitic rocks and silty dolomites. Preserved sedimentary features in the meta-siliciclastic rocks include cross-bedding, ripples and mud drapes that are consistent with a shallow, tidally-influenced marine depositional setting. Three subunits (SG1, SG2 and SG3) were identified within the Serra do Garrote Formation based on molar Al/Ti ratios. Samples from the Serra do Poço Verde Formation have Al/Ti ratios similar to Serra do Garrote samples, indicating they shared similar sediment sources. Geochemical indicators of source rock composition and tectonic discrimination plots (Zr/Sc versus Th/Sc, Th – Sc – Zr/10, Hf versus La/Th, and rare earth element) revealed that andesitic to felsic continental arc rocks were the major source of detritus.The majority of concordant detrital zircons are Paleoproterozoic in age, with 207Pb/206Pb dates between 2350 and 1850 Ma, which overlap with the early and late stages of the Trans-Amazonian orogeny (≈2.3–1.9 Ga) and are coeval with construction of the Paleo-Mesoproterozoic Columbia Supercontinent. Detrital zircon provenance patterns suggest a rift-related tectonic setting during deposition. The youngest concordant U – Pb zircon 207Pb/206Pb dates (≈1340 Ma) are coeval with the formation of regional bimodal volcano – sedimentary sequences and layered that are interpreted to have formed during continental rifting. These events are synchronous with increased tectonic and magmatic activity (Mackenzie dyke swarm, North America; mafic dyke swarms, North China Craton) related to the break-up of the Columbia Supercontinent.

Diagenetic controls on the quality of the Middle Permian Lucaogou Formation tight reservoir, southeastern Junggar Basin, northwestern China

The Middle Permian Lucaogou Formation, in the southeastern part of the Junggar Basin in China, is a typical tight oil reservoir with low to medium porosity (average 11.2%) and ultra-low permeability (average 0.01 mD). Here, we investigate Lucaogu Formation reservoir diagenesis and its impact on reservoir quality to effectively explore, appraise, and develop this complex tight oil reservoir. Core and thin section analyses, scanning electron microscopy, X-ray diffraction (XRD), electron probe analyses, fluid inclusions, isotopic analysis, pressure-controlled mercury injection, rate-controlled mercury injection, and micro-nanometer X-ray computer tomography (CT) imaging were performed on core samples from the Lucaogou tight reservoir. The sweet spots in the Lucaogu Formation are composed of siltstone, tuffaceous siltstone, and silty dolomite, which underwent two stages of carbonate cementation (primarily calcite, dolomite, and ankerite), clay mineral alteration and dissolution, and one stage of quartz cementation. The major authigenic mineral cements (derived from the original formation water, clay mineral alteration, dissolution of acid-soluble components, and the compaction and drainage of the interbedded mudstone) are also a probable source of ions. It is likely that oil emplacement occurred after quartz cementation and the first stage of carbonate cementation and feldspar dissolution, but before the second stage of calcite cementation and feldspar dissolution. Organic acids produced during hydrocarbon generation contributed to the dissolution of the feldspar and tuffaceous components, locally preventing a decrease in porosity by concurrent cementation and compaction. Carbonate cementation decreased the porosity and permeability of the silty dolomite substantially; in the siltstone, decreased permeability is largely attributed to cementation of clay minerals. The pore structure of the sweet spot is controlled not only by the various cementation events, but also by other geological factors, such as heterogeneous dissolution of the tuffaceous component.

Indication of rare earth element characteristics to dolomite petrogenesis—A case study of the fifth member of Ordovician Majiagou Formation in the Ordos Basin, central China

Abstract Rare earth elements (REEs), as important geochemical tracers, can well reflect the information carried by diagenetic fluids. In this paper, petrological and REE characteristics are studied to investigate diagenetic fluid information and dolomite petrogenesis using samples from the fifth member of Ordovician Majiagou Formation (Ma5) in the Ordos Basin based on the comprehensive analysis of C, O, and Sr isotopes. Three types of dolomites are identified in the study area, namely the dolomicrite, micrite-silty dolomite, and silty dolomite. Different types of dolomites have distinct total REE contents, among which the dolomicrite has the highest average total REE content and that could be possibly attributed to the abundant algal and argillaceous materials. All the three types of dolomites are featured by lower total contents of REEs than marine micrite, enriched light rare earth elements (LREEs) and deficit heavy rare earth elements (HREEs), which could result from the constant diffusion of REEs in the late diagenetic environment as well as the stronger complexing capacity of HREEs than LREEs. REE partition patterns in three types of dolomites are similar to those in marine micrite, which indicate the dolomitization fluids to be seawater or marine-origin fluids. In the dolomicrite, an obvious negative Eu anomaly (average value 0.63) and a faint negative Ce anomaly (average value 0.94) suggest that the sedimentary environment should be featured by low temperature and weak oxidation. Meanwhile, negative δ13C excursion (average value - 1.23‰), positive δ18O excursion (average value −7.59‰), and high mean 87Sr/86Sr ratios (average value 0.70992) along with petrological characteristics demonstrate the dolomicrite to originate from penecontemporaneous dolomitization. In the micrite-silty dolomite, the obvious negative Eu anomaly (average value 0.69) and the medium negative Ce anomaly (average value 0.72) indicate such kind of dolomites should be formed in the sedimentary environment characterized by low temperature and weak oxidation-weak reduction. The negative δ13C excursion (average value −0.43‰), positive δ18O excursion (average value −7.29‰), and relatively low mean 87Sr/86Sr ratios (average value 0.70955) identify the micrite-silty dolomite to be product of seepage reflux dolomitization in the supratidal zone. In the silty dolomite, the weakened negative Eu anomaly (average value 0.75) indicates relatively high temperature, and the obvious negative Ce anomaly (average value 0.66) implies reducing environment. Based on analysis of values of δ13C (average value −0.65‰) and δ18O (average value −6.46‰) as well as ratios of 87Sr/86Sr (average value 0.70958), silty dolomite is regarded to form due to burial dolomitization.

The origin and formation model of Permian dolostones on the northwestern margin of Junggar Basin, China

This study investigates the mechanism of dolostone formation and establishes a dolomitization model in the Permian strata on the northwestern margin of Junggar Basin, China. Dolomitic rock samples are collected from the Permian Fengcheng Formation in Urho area and then characterized by petrological, mineralogical, carbon and oxygen isotope, and trace element geochemical analyses. Results show that the major types of dolomitic rocks include dolomitic mudstone, dolomitic siltstone, dolomitized tuffaceous siltstone, and dolomitized tuffaceous mudstone. The dolomitized rocks are dominated by euhedral or subhedral powder- and fine-crystal dolomites formed by replacement lacustrine argilla-calcareous and siliceous (tuffaceous) components and commonly filled with residual and late calcite cements. The parameters of dolomitic rocks show great variations, including the V/Ni ratio (1.02–4.88), Sr content (95.9–783.6μg/g), Mg/Ca ratio (0.68–5.13), degree of ordering (0.39–1.00), δ18OPDB (−14.8‰ to 3.2‰), and δ13CPDB (−1‰ to 5.2‰). The dolomitic rocks have multi-stage origins and were formed in a semi-closed continental brackish-saltwater bay with weak hydrodynamic processes, deep water bodies, and relatively quiet conditions. In the Permian depositional stage, a combination of complex tectonic activities, fault development, hot subtropical climate, and frequent volcanic activities provided not only Mg2+ source for dolomitization but also channels for rapid flow and seepage of Mg-rich fluids. The origins of dolostones in the study area include penecontemporaneous dolomitization, burial dolomitization, and hydrothermal dolomitization. This study lays a foundation for further studies on dolomite formation and dolomite reservoir, and provides effective methods for researching complex dolostone (tuffaceous, shale and silty dolomite) formation.

The Padaeng Supergene Nonsulfide Zinc Deposit, Mae Sod, Thailand

The Padaeng deposit near Mae Sod in western Thailand was the first supergene nonsulfide zinc deposit in the world to be developed as a large modern mining operation. The mine and associated zinc smelter, operated by Padaeng Industry Public Company Ltd. since 1984, went into production with reserves of 4.59 Mt at a grade of 28.9 percent zinc with a 10 percent zinc cutoff. Current resources are 5.14 Mt at a grade of 12.0 percent zinc with a 3 percent zinc cutoff. The Padaeng deposit is hosted by a mixed carbonate-clastic sequence of Middle Jurassic age. The deposit occurs in the hanging wall of the Padaeng fault, a major northwest-trending structure that was active through Cretaceous and Tertiary tectonism and uplift. Nonsulfide zinc ore comprises dominant hemimorphite with minor smithsonite and hydrozincite. Strata-bound ore zones occur within a northwest-dipping, deeply weathered, dolomitic sandstone; steeply dipping and irregular karstic zones in underlying massive, silty dolomite are controlled by north-trending fracture zones. Sulfide zinc-lead mineralization of Mississippi Valley type occured extensively in the vicinity of the Padaeng mine, most notably the small resources at Pha De and Hua Lon. Mineral deposits are typically sphalerite rich with minor galena and pyrite, forming small-scale open-space fillings, veins, and replacements within hydrothermal dolomite. Mineralization is dominantly strata bound within a horizon of intense hydrothermal dolomitization that forms the stratigraphic hanging wall to the nonsulfide ore zones at Padaeng. The only significant sulfide at the Padaeng mine is within this unit. Only trace sulfide occurs peripheral to, or down dip of, strata-bound or steeply dipping, nonsulfide orebodies. Sulfide mineralization is believed to have accompanied Cretaceous uplift and deformation, related to the onset of oblique subduction beneath the western margin of the Shan-Thai terrane. The nonsulfide deposit is believed to have formed when a substantial body of sulfide ore was uplifted on the margin of the Mae Sod Tertiary intermontane basin, commencing in the middle to late Miocene. Zinc-bearing acidic supergene fluids, generated by oxidation of the precursor sulfide body, reacted with carbonate in the underlying stratigraphic section to precipitate hemimorphite and smithsonite. Fluids were channeled by permeable dolomitic sandstones and by steep fracture and fault zones. Acidic fluids promoted deep weathering and karst formation, allowing mineralization to extend down dip in sandstone units for at least 150 m and vertically for a similar distance in steep structural zones. Transport of zinc out of the precursor sulfide body was facilitated by a falling water table, owing to uplift of the Padaeng fault block and a change from wet tropical to monsoonal or semiarid climatic conditions. There is no evidence for significant in situ replacement of sulfide deposits, and the leached remnants of the precursor sulfide body have been removed by erosion. The supergene process of dissolution and reprecipitation of zinc in the host rocks increased zinc grades and separation of zinc from lead, producing an economically attractive deposit. Successful exploration for this type of deposit requires a good understanding of the controls on primary sulfide mineralization and a good knowledge of local neotectonism, uplift history, hydrogeology, climatic evolution, and weathering history.

Pearsall (Aptian Cretaceous) Subsurface to Outcrop Sequence Stratigraphy, Central Texas: ABSTRACT

ABSTRACT Bexar (Upper Pearsall) Shale is a widespread subsurface body of marine clay mudstone; in south-central Texas it disappears updip along a line through central Medina and Bexar Counties and the northwestern edges of Guadalupe, Caldwell, and Bastrop Counties. Bexar Shale overlies calichified Cow Creek Limestone and underlies Hensel Dolomite; in outcrop Hensel Dolomite rests on calichified Cow Creek. The Cow Creek and other mid-Pearsall limestones are local high-stand deposits; the basal Bexar is a transgressive bed and the bulk of the unit a shoaling-upward ramp deposit. Hensel silty dolomite (laminated to churned, with glauconite, collophane, beekite, and milky quartz geodes) and overlying Lower Glen Rose grainstones (containing widespread subarkose sand grains) are a high-stand lagoon/bar sequence that overlaid Bexar Shale and spread 25 km inland from the Bexar pinchout. The Hensel/basal Glen Rose shoaling sequence is the earliest of three in the Lower Glen Rose. Type Hensel Sandstone around Lake Travis was deposited (from local flash floods) after 35 m of marine Bexar shale and 35-50 m of Glen Rose carbonates farther south (an ammonite zone later). Deep red clay between calichified Cow Creek and Hensel Sandstone around Lake Travis may be remnant Bexar Shale or may be local and unrelated. Actual eustatic/tectonic/sediment-supply history does not fit a simple sequence-stratigraphic pattern.

Interreef crinoid fauna from the Mississinewa Shale Member of the Wabash Formation (Northern Indiana; Silurian; Echinodermata)

A true interreef crinoid fauna lacking components of reefal facies is reported from the Ludlovian of the Mississinewa Shale Member of the Wabash Formation in northern Indiana. The Mississinewa facies that contains this fauna is a dolomitic siltstone or a silty dolomite. The crinoid association reported is composed entirely of new species and is different from other reef and reef-associated, interreef, and non-reef crinoid associations. New species described, herein, include the following: Dimerocrinites (Eucrinus) ankylos n. sp., Periechocrinus shaveri n. sp., Bohemicocrinus? occidens n. sp., Macrostylocrinus styo n. sp., M. homalos n. sp., Gissocrinus hypselos n. sp., and Dendrocrinus diaphoros n. sp. This may be the first occurrence of Bohemicocrinus from outside Bohemia.

Results of Deep Exploratory Drilling Between Long and Newark Valleys, White Pine County, Nevada-Implications for Oil Migration in the Nearby Yankee Gold Mine Paleohydrothermal System

In mid-1992, a consortium headed by Pioneer Oil and Gas (Midvale, Utah) drilled a deep (6700 ft) exploratory well in the southern Ruby Mountains-Buck Mountain are near the Alligator Ridge mining district in White Pine County, Nevada. The test well is located 1.5 mi southwest of USMX, Inc.'s, Yankee gold mine, an open-pit operation centered on a Carlin-type, sediment-hosted gold orebody noteworthy for containing abundant, fracture-controlled live oil. The Pioneer well as dry, but intersected much of the same stratigraphic section hosting gold at Yankee, thereby providing valuable clues to mechanisms of oil migration at this unusual, oil-bearing precious-metal deposit. Most of the gold at Yankee is hosted by the Devonian Pilot Shale, with a basal argillaceous limestone containing the bulk of the deposit's live oil. The equivalent section in the Pioneer wildcat well is a silty calcareous dolomite. Whereas the basal Pilot limestone at Yankee is rich in thick, locally gold- and arsenic-anomalous calcite veins and modules hosting abundant oil-bearing fluid inclusion, the basal Pilot dolomite in the Pioneer well contains only a few thin calcite-pyrite veinlets devoid of fluid inclusions. Moreover, the Yankee calcite veins have the same light-stable-isotope signatures as hydrothermal carbonate veins near or elsewhere inmore » the Alligator Ridge district. These relationships imply that oil at Yankee migrated in the same hydrothermal system responsible for gold mineralization. Such systems elsewhere in the eastern Basin and Range, given favorable source rocks, traps, seals, and migratory pathways, might well have formed not only gold deposits, but also rich, spatially coincident oil reservoirs.« less

Origin of Sulfide and Phosphate Deposits in Upper Proterozoic Carbonate Strata, Irece Basin, Bahia, Brazil

Carbonate strata of the Una Group represent late Proterozoic platform sedimentation in the Irece basin of north-central Brazil. Stratabound sulfide- and phosphate-rich units occur within a 50-m thick tidal flat sequence of dolomitic limestone and cherty dolomite. Three types of primary phosphate concentrations are present: columnar stromatolitic, laminar stromatolitic, and intraclastic. Resedimented phosphate clasts and phosphatic units interbedded with non phosphatic dolomites suggest early diagenetic replacement of algal carbonate units. Local stratabound Zn-Pb-Ag sulfide concentrations at the Tres Irmas prospect occur within silty dolomite with shallow water sedimentary structures and local disturbed laminae, synsedimentary faults, and breccias. Sulfide minerals include pyrite, sphalerite, galena, marcasite, jordanite, tetrahedrite, and covellite. Pyrite crystal aggregates commonly show bladed forms. Nodular aggregates of length-slow quartz are locally associated with sulfides. Sulfur isotope analyses indicate relatively uniform heavy {delta}{sup 34}S values. Barite shows a {delta}{sup 34}S range from +25.2 to +29.6{per thousand}, CDT. Pyrite and sphalerite representative of a variety of textural types have a {delta}{sup 34}S range of +20.2 to +22.6{per thousand}. Late Proterozoic evaporite sulfates show a wide range of {delta}{sup 34} S values from about +10 to +28{per thousand}. Thus, the {delta}{sup 34}S values for Irece barite could reflect original seawater sulfate values.more » However, the relatively heavy {delta}{sup 34}S values of the associated sulfides suggests that the original seawater sulfate was modified by bacterial sulfate reduction processes in shallow sea floor sediments. Textural and {delta}{sup 34}S evidence suggests that a later stage of metallic mineralization scavenged sulfur from preexisting sulfides or from direct reduction of evaporitic sulfate minerals.« less

Precambrian cyclic rhythmites: solar-climatic or tidal signatures ?

For more than 60 years geologists have sought evidence of solar-climatic cyclicity in rhythmically laminated sedimentary rocks, but claims in general have not been persuasive. Three Precambrian rhythmite sequences in Australia that comprise varve-like laminae recently have received attention, however, as their conspicuous cycles ofca. 10-14 and/or 20-25 laminae have been ascribed a sunspot-cycle origin. They are the 2500 Ma old Weeli Wolli Formation, the 1750 Ma old Wollogorang Formation, and the 650 Ma old Elatina Formation. New observations for the Weeli Wolli Formation, a siliceous banded iron-formation, suggest a cycle period exceeding the 23 microband couplets proposed by Trendall, casting doubt on the solar interpretation. The Weeli Wolli cyclicity may record Earth-tidal rhythms that modulated the discharge and composition of silica- and iron-bearing fumarolic waters. The structure of the cycles of silty dolomite and mudstone in the Wollogorang Formation does not support a sunspot-cycle origin, and a tidal control on sedimentation should be considered. The Elatina sequence of cyclic sandstone and silts tone laminae displays several empirical similarities to the sunspot series. The discovery of thicker, more complex lamina-cycles in the correlative Reynella Siltstone has, however, caused reappraisal of the solar interpretation of the Elatina rhythmites. The Reynella cycles consistently contain 14 or 15 laminae, with many laminae comprising pairs of‘semi-laminae’; the overall structure of the cycles is similar to the mixed (semidiurnal and diurnal) fortnightly tidal growth patterns of modern bivalves. Also, recent observations that fine material can be transported in suspension to deeper waters offshore by ebb-tidal currents provides a tidal mechanism that can explain the deposition of thin, laterally extensive, graded laminae whose thickness would be a measure of tidal range and current speed. By this tidal model, the Elatina laminae are regarded as diurnal increments, and the lamina-cycles as commonly abbreviated fortnightly groupings. The Elatina series so interpreted may encode unique information on lunar orbital periods and the Earth’s palaeorotation: the data indicateca. 30.5 days per lunar month, 13.1 lunar months andca. 400 days per year, and lunar apsides and lunar nodal cycles of 9.7 andca. 19.5 years respectively some 650 Ma ago. Evidence for significant modulation of terrestrial climate by the solar activity cycle in the geological past may prove as elusive as have convincing indications of solar signals in modern patterns of weather and climate.

Yates Field, West Texas: Thousands of Caves, Millions of Years, Billions of Barrels: ABSTRACT

Since its discovery in 1926, the Yates field has produced 1.04 billion bbl of oil from an estimated original oil in place of approximately 4 billion bbl. The reservoir produces from Upper Permian dolomites, siltstones, and sandstones at depths from 1,000 to 1,900 ft (305 to 580 m). Wells with extraordinary flow rates were common during the early life of the field. Of the 636 wells drilled before unitization of the field in 1976, 203 potentialed for more than 10,000 BOPD, 26 for more than 80,000 BOPD, and one for more than 200,000 BOPD. These remarkable early productivities were due to several geologic factors: (1) reservoir pressures that were considerably in excess of those needed to support flowing wells; (2) an immense network of reservoir pores, ranging in size from micropores to small caves, tied together by open fractures; and (3) an oil column that, at discovery, filled at least 350 ft (107 m) of closure in the broad Yates dome, which covers an area of 35 mi2 (91 km2). Stratigraphic units in the Yates field reservoir are of Guadalupian (Kazanian) age. The units consist of the upper San Andres dolomite (the principal reservoir unit), overlain unconformably by Grayburg siltstones and silty dolomites; Queen dolomitic siltstones and sandstones; and lenses of dolomitic sandstone in the basal Seven Rivers, the formation whose thick anhydrites complete the trap. The San Andres is comprised of two major depositional facies. The first, characteristic of the formation in westside Yates, is an intertidal and lagoonal unit of compact dolomitic mudstones and wackestones and bedded clay shales. The second is dominated by porous, subtidal grain-supported bioclastic dolomites rich in fusulinids and calcareous algae, and is typical of the formation in the eastside. he facies boundary between eastside and westside in the San Andres moved eastward through time and thus displays the prograding relationships found in many fields along the margins of the Central Basin platform in the Permian basin of west Texas and southeast New Mexico. One small framestone reef composed mostly of colonial corals and Tubiphytes has been recognized in core from the upper San Andres in a well at the eastern edge of the field. Much of the present structure on the reservoir was developed during the latest Permian and may be related to renewed movement along deep faults which had been active during the Pennsylvanian and Early Permian. In the upper 60 ft (18 m) of the San Andres, chiefly in eastside Yates, evidence of karstification is abundant, including banded cave cements, cave sediments, collapse breccias, and solution pores ranging in size from small vugs to caves. More than 300 San Andres caves have been detected by wireline logs or as bit drops, and must represent a cave population numbering in the thousands. Most of the caves and other karst features were developed by processes associated with dynamic freshwater lenses beneath a cluster of islands formed when Late Permian lowering (or lowerings) of sea level exposed the upper San Andres, then limestone, to rainfall and dissolution. Evidences of karst are also present in the Grayburg dolomites but are less common than in the San Andres. They probably represen local subaerial exposure of relatively short duration. Infill drilling, logging, and coring since unitization have greatly enhanced geologic knowledge about the Yates field reservoir. This knowledge is being combined in many ways with engineering data and techniques to support management of the reservoir during the remaining field life. End_of_Article - Last_Page 1439------------

Subsurface Study of Lower Ordovician Prairie du Chien Group and Underlying Cambrian Formations and Their Relation to Pre-Glenwood Unconformity in Southern Peninsula of Michigan: ABSTRACT

Prior to discovery of commercial gas in Lower Ordovician Prairie du Chien sandstones at Falmouth field, Missaukee County, in April 1981, only 119 wells had reached underlying Cambrian rocks in Michigan's Southern Peninsula. The resulting deep basin drilling activity to January 1, 1987, has resulted in 17 Prairie du Chien gas discoveries in the north-central part of the Michigan basin and an additional 46 Cambrian penetrations. This study also utilized 235 Prairie du Chien partial penetrations. It is now possible to correlate with relative certainty the standard Cambrian formations of the subsurface of southern Michigan to wells within and on the other margins of the basin. These, in ascending order, are Mount Simon, Eau Claire, Dresbach, Franconia, and Trempealeau. Because of the previous lack of control, these units were commonly misidentified in northern wells in previous studies. The present study largely corroborates previous recent work. Four ascending formations of the Prairie du Chien Group were also correlated throughout the basin: Umlor, Foster, Bruggers, and Goodwell (newly proposed). Two type wells demonstrate the character of the Goodwell Formation. The Bruggers Sandstones has been further divided into four informal zones, based on extending correlations of thin, silty dolomite marker beds on gamma-raymore » logs from the Goodwell and Woodville fields area, Newaygo County. Gas is produced from these Brugger zones. Twelve detailed stratigraphic cross sections (using 183 wells) and a pre-Glenwood paleogeologic map demonstrate marked basin-wide pre-Middle Ordovician erosional truncation.« less

Stratigraphy, Depositional Environments, and Diagenesis Related to Porosity Development and Destruction in Jefferson Formation, Northwestern Montana: ABSTRACT

The Jefferson Formation in northwestern Montana consists of a lower member (< 155 m or 508 ft) and the Birdbear Member (< 72 m or 236 ft), which are correlative to the Duperow and Birdbear Formations of the Williston basin. The lower member consists of sucrosic and nonsucrosic dolomites, limestones, and evaporite solution breccias. The Birdbear Member consists of silty nonsucrosic dolomites and limestones, and sucrosic evaporite solution breccias. Paragenesis of diagenetic events based on petrographic cross-cutting relations, cathodoluminescence, microprobe analysis, conodont alteration indices, and field relations indicate that most porosity development and destruction occurred at relatively shallow depth prior to thrusting. Devonian events include early cementation of desiccation structures and intergranular spaces, localized vuggy and moldic porosity development, and dolomitization and formation of intercrystalline porosity. Mississippian to pre-Jurassic events include burial to oil window, kerogen maturation, and loading stylolitization. Later erosion of Mississippian strata and ground-water circulation resulted in brecciation, subvertical fracture development related to collapse, and dedolomitization followed by near-complete calcite cement infilling of available porosity. Paleocene to early Eocene thrusting events include stylolitization, folding of breccias, and nonfilled fracture formation. Post-early Eocene exposure-induced events results in vuggy porosity and precipitation of speletic cements.