Moderate Burial Depths Research Articles

Preliminary evaluation of gas content of the No. 2 coal seam in the Yanchuannan area, southeast Ordos basin, China

Gas contents are highly variable in coalbed methane (CBM) reservoirs of the Yanchuannan (YCN) area of the southeast (SE) Ordos Basin, China. We used diverse geologic data derived from more than five years of exploration to provide insight into the origin of this variability and the consequences of gas content on reservoir performance. Major factors affecting gas content variability include gas generation, migration, trapping and preservation. Gas generation affects gas content variability on the scale of the total resource, whereas gas migration influences the inhomogeneous redistribution of gas content on a regional or local scale. Gas trapping and preservation affect the “as-observed” content. The potential for high gas content is controlled directly by the composite result of gas generation, migration, trapping and preservation. CBM in the YCN area is produced from the relatively thick seam (~2.09 m and 8.05 m, with an average of 5.97 m) that is distributed through 450–1200 m of the stratigraphic section. Gas content tends to be structurally and hydrodynamically controlled in the order of simple structure (folds and small faults)>complex structure (large regional faults) and groundwater stagnant zones>runoff zones. Coal samples in the YCN area typically have Langmuir volumes between 31.86 and 46.51 cm3/g, which correlates with coal rank. Reservoir heterogeneity including coal composition, pore structure and matrix moisture content may contribute to the heterogeneous gas content. Gas content is generally high where hydrodynamic trapping of gases occurs and may be anomalously low in areas of active recharge with downward flow potential and/or convergent flow where there is no mechanism for entrapment. In the YCN area, the most favorable area for CBM exploration and development is in the center block (block B), where great coal thickness, moderate burial depth, favorable hydrodynamics and an anticlinal trap coincide to yield high gas contents.

Evaluating the mechanical compaction of quartzarenites: The importance of sorting (Llanos foreland basin, Colombia)

Mechanical compaction is the main porosity-reducing process in sandstones, including high-reservoir-quality rigid-grain sandstones. For such sandstones, the extrapolation of theoretical or experimental compaction algorithms needs calibration with rocks having well constrained burial histories. Evaluating the compaction of these rocks is achieved by comparing current intergranular volume (IGV) with depositional IGV, which is strongly dependent on sorting. However, because sandstone sorting is difficult to measure accurately, its impact on depositional porosity and compaction state is largely underestimated. We use the quartzarenites of the Oligocene Carbonera Formation in the subsurface of the hydrostatically-pressured Llanos basin to illustrate the importance of sorting when evaluating the compaction of rigid-grain sandstones. IGV and sorting were measured in core samples using a combination of transmitted-light and cathodoluminescence images, resulting in improved accuracy over standard procedures. The compaction state of clean quartzarenites at given depths is best described using IGV-versus-sorting plots, which are used to derive compaction curves for specified sorting values. The IGV-versus-sorting trends are displaced to lower IGV values with increasing burial depth. The differences in IGV caused by differences in sorting exceed the differences in IGV resulting from 1000 m of burial, illustrating the high impact of sorting when evaluating compaction. Contrasting with published experimental results, the compaction of the Llanos basin ductile-grain-poor quartzarenites is independent of grain size, and grain rearrangement is the main compaction mechanism during the first ∼1.6 km of burial. Based on the Llanos data, we have generated IGV-versus-depth curves for clean pure quartzarenites of specific sorting, which can be used to predict their maximum primary porosity up to moderate burial depths. Differences with other published burial curves are probably related to unaccounted variations in sorting, ductile-grain content and framework-strengthening cements. However, the Llanos basin quartzarenites contain virtually no cements, explaining their high degree of compaction relative to other rigid-grain sandstones, and making them ideal to isolate the effects of compaction on the IGV of quartzarenites. The Llanos basin data suggest that, below ∼2.5 km of depth, clean well- to moderately well sorted quartzarenites continue reducing their IGV by mechanical compaction below the 26% limit, which should apply only to extremely well sorted, rigid grain, uncemented sandstones.

Insights from petrography, mineralogy and U–Pb zircon geochronology into the provenance and reservoir potential of Cenozoic siliciclastic depositional systems supplying the northern margin of the Eastern Black Sea

The Eastern Black Sea is one of the few remaining underexplored hydrocarbon basins in Europe. Reservoir quality is a key risk factor in the basin. The analysis of up-dip portions of Oligocene to Pliocene depositional systems exposed around its northern and eastern margins enable the composition and provenance of sandstones within the basin to be predicted. Two major sources of quartz-rich sandstone are present: the Russian western Greater Caucasus, which became a subaerial sediment source in the Oligocene, and the East European Craton–Scythian Platform. These are predicted to form better quality reservoirs at shallow and moderate burial depths than their rock fragment-rich counterparts, due to lesser amounts of compactional porosity loss. Oligo-Miocene sandstones from the Russian western Greater Caucasus were deposited as turbidites in the Tuapse Trough. Sandstones from the East European Craton–Scythian Platform were largely trapped within the Indolo-Kuban Basin north of the Greater Caucasus. They only entered the Eastern Black Sea in large volumes from latest Miocene or Pliocene time in the region of the Kerch–Taman shelf at the western tip of the Greater Caucasus. However, it is possible that a precursor to this system supplied the Oligocene to Early Miocene reservoir sandstones within the Subbotina field. The Dziruli Massif, in western Georgia, also generated quartz-rich sandstones, but these are not thought to have entered the Black Sea.

Climatic and diagenetic signals in the stable isotope geochemistry of dolomitic paleosols spanning the Paleocene–Eocene boundary

The precipitation of primary dolomite in soil is rare and controversial, and its occurrence indicates highly unusual geochemical and climatic conditions. We utilize isotopic and petrographic techniques, including clumped isotope thermometry, to evaluate diagenetic alteration and preservation of primary climatic signal in dolomitic paleosol nodules from a section spanning the Paleocene–Eocene boundary in central Utah. Isotopic measurements differentiate samples into groups that may be interpreted in terms of alteration; however moderate burial depths and extreme warmth during the interval of soil formation make it difficult to reach definitive conclusions from isotope values alone. With the addition of petrographic analysis, including cathodoluminescence and scanning electron microscopy, we are able to identify textural differences between samples suggesting differing amounts of recrystallization. We conclude that while some nodules have experienced significant diagenetic alteration the original nodule mineralogy is dolomite, and the majority of sampled nodules retain a primary paleoclimate signal. Early Paleogene temperatures recorded at this site are considerably warmer than those suggested for the continental interior from alternate proxies, a result that may be partially the result of the clumped isotope temperatures recording warm season soil temperatures, rather than mean annual air temperature. Soil water isotopic composition is suggestive of heightened aridity, particularly through the Paleocene–Eocene Thermal Maximum (PETM).

Impact of regional variation in detrital mineral composition on reservoir quality in deep to ultradeep lower Miocene sandstones, western Gulf of Mexico

Future exploration in lower Miocene sandstones in the Gulf of Mexico will focus increasingly at depths greater than 4.5 km, and reservoir quality will be a critical risk factor in these deep to ultradeep reservoirs. The goal of this study was to understand the variation in reservoir quality of lower Miocene sandstones across the western Gulf of Mexico. To do this, we examined regional variation in detrital mineral composition, diagenesis, and reservoir quality in five areas: (1) Louisiana, (2) upper Texas coast, (3) lower Texas coast, (4) Burgos Basin, Mexico, and (5) Veracruz Basin, Mexico using petrographic and petrophysical data from depths of 0.9–7.2 km.There are strong variations in mineralogical composition within the study area. Lower Miocene sandstones from offshore Louisiana have an average composition of quartz = 86%, feldspar = 12%, and rock fragments = 2% (Q86F12R2). Feldspar and rock-fragment content increase southward as source areas shifted to include volcanic and carbonate rocks. Composition of samples from offshore Texas ranges from Q67F24R9 in the upper Texas coast to Q58F24R19 in the lower Texas coast. Lower Miocene sandstones from the onshore Burgos Basin, northern Mexico, have an average composition of Q54F22R23, whereas sandstones from the Veracruz Basin, southern Mexico, contain the highest proportion of rock fragments, Q33F12R55. Main diagenetic events in quartz-rich lower Miocene sandstones in Louisiana were mechanical compaction and precipitation of quartz cement. Compactional porosity loss increased to the south with increasing rock-fragment content. Calcite is the most abundant cement in the south and is strongly related to reservoir quality loss.At moderate burial depths, the best reservoir quality occurs in quartz-rich sandstones in Louisiana and decreases with increasing lithic content in Texas and Mexico. Porosity is higher in Louisiana and upper Texas than in lower Texas and Mexico at all depths and temperatures, but at depths >5 km and temperatures >175 °C, porosity differences are lessened. The lower Miocene sandstone trend in the western Gulf of Mexico from Louisiana to Mexico is an example of the importance of variation in detrital mineralogy as a control on diagenesis and reservoir quality.

IMPACT OF DIAGENESIS ON RESERVOIR QUALITY IN RAMP CARBONATES: GIALO FORMATION (MIDDLE EOCENE), SIRT BASIN, LIBYA

The reservoir quality of Middle Eocene carbonates in the intracratonic Sirt Basin is strongly influenced by depositional facies and various diagenetic processes. Based on data from core samples and well logs from five boreholes in the Assumood and Sahl gasfields in the subsurface of the north‐central Sirt Basin, six major carbonate facies (and fourteen microfacies) are distinguished in the Middle Eocene Gialo Formation (thickness ∼1100 ft/335 m). Wackestones‐packstones dominate the Gialo Formation with abundant grains of larger benthic foraminifera, especially nummulites, in a matrix composed mostly of finely comminuted bioclastic material. Sediments were deposited as a mosaic of facies on a broad carbonate ramp in moderate‐energy nummulite‐packstone banks, with locally restricted back‐bank lagoons and fore‐bank areas of foraminiferal debris passing to offshore lime mudstones.Marine diagenesis was minor with micritization of bioclasts and rare vadose marine fibrous cements. Sparry calcite cements are ubiquitous and were precipitated during shallow to moderate burial from seawater and/or meteoric water, the latter largely unaffected by surface‐pedogenic processes. This is indicated by the fabrics and pre‐ and post‐compaction precipitation of drusy spar and echinoderm overgrowths, and supported by their bright‐to‐dull luminescence and low negative δ18O signatures (‐2.68 to ‐4.16%° PDB). Some early neomorphic alteration of calcitic bioclasts is suggested by bright CL and marine to low negative δ18O values (‐1.06 to –3.93%° PDB). Bioclastic grains have δ13C values ranging between 0.76 and 1.19%° PDB, interpreted as marine signatures. Similar low positive δ13C values of the cements (0.9 to 1.05 %°) indicate a source of carbonate from dissolution of grains and/or seawater/meteoric water, but without any near‐surface/soil effects. Dissolution of grains and matrix, notably originally aragonitic grains but also the originally high‐Mg calcite nummulites, was a major porosity‐enhancing process, and took place from shallow to moderate burial depths. Fractures may locally have increased porosity too.Shallow‐water packstones/rudstones containing both primary intergranular and secondary biomouldic porosity have the best reservoir quality and these are concentrated in the upper parts (top ∼100 ft/30 m) of the wells, with porosity‐permeability decreasing downwards. This trend relates to the broad, large‐scale facies pattern of more mud‐dominated facies giving way upwards to more grainy, nummulite‐dominated facies as a result of an overall shallowing of the depositional environment. In addition, the influx of meteoric waters in the upper part of the Gialo platform, before drowning and deposition of the overlying Augila Shale, increased porosity through dissolution. This integrated study has helped in understanding the reservoir heterogeneity and hydrocarbon potential of the Gialo carbonates.

The application of an integrated approach in exploration of lacustrine turbidites in Jiyang Sub-basin, Bohai Bay Basin, China

This study presents an integrated approach that has been successfully used in delineation of turbidite reservoirs in the Jiyang Sub-basin, Bohai Bay Basin, China. The techniques involve an integration of multi-disciplinary investigations, which comprise firstly the determination of depositional areas of turbidites using a sequence stratigraphic approach, then mapping the spatial distribution of turbidites using seismic velocity data, and finally, the selection of drilling prospects by an optimization process using available well logs and core data. Interpreted turbidites with a moderate burial depth and thickness of at least 10 m were targeted for drilling. The integrated approach has been proved to be quite successful in recent exploration activities with approximately 60 million tonnes (438 million bbls) of proven in place oil reserves had been discovered for the 6-year period of 1996 to 2001 in turbidite reservoirs, highlighting the importance of turbidite plays in current and future exploration.

Abstract: Formation and Stability of Kaolinite in Subarkosic Sandstones at Low to Moderate Burial Depth 

Abstract: Formation and Stability of Kaolinite in Subarkosic Sandstones at Low to Moderate Burial Depth 

Lithologic and Diagenetic Controls on Intergranular Porosity of the Ivishak Sandstone, Northstar Unit, Beaufort Sea, Alaska: ABSTRACT

The Triassic lvishak Sandstone has a bimodal pore system consisting of (1) intergranular pores and (2) intragranular microporosity within white chert grains. Capillary modeling indicates that throughout the Ivishak at Northstar (offshore Beaufort Sea) microporous chert is water saturated, and only the intergranular pore system is available to hydrocarbons. The volume of intergranular porosity is largely related to depositional sorting. Well-sorted sandstones retain an average of 4% greater intergranular porosity than poorly-sorted sandy conglomerates despite extensive diagenesis. Paragenesis follows the sequence: pyrite > apatite > siderite > siderite dissolution > quartz > kaolinite > barite > calcite. Early diagenetic pyrite is concentrated near the top of the Ivishak, directly below the organic-rich, marine Shublik Shale, and is a result of bactedal sulfate reduction in the presence of marine pore fluids. Siderite cement has depleted [delta][sup 18]O[sub PDB] (-5.9 to -6.6 [per thousand]) and [delta][sup 13]C (-3.4 to -8.3[per thousand]) and highly radiogenic [sup 87]Sr/[sup 86]Sr (0.71064), reflecting the influx of meteoric waters and precipitation at 25 -45[degrees]C. Partial dissolution of siderite cement restores only minor amounts of porosity. Continued invasion by meteoric water led to the formation of abundant quartz cement ([approximately]7.5%) and associated kaolinite ([approximately]2%). Calcite forms a pervasivemore » cement in discrete 4 - 25 foot thick beds and is characterized by moderately depleted [delta][sup 18]O[sub PDB] (-6.3 to -7.1 [per thousand]), positive [delta][sup 13]C (0.9 to 4.3 01.), and highly radiogenic [sup 87]Sr/[sup 86]Sr (0.71088). These isotopic data are consistent with precipitation from continentally derived meteoric waters at moderate burial depths.« less

Fluid Inclusion Evidence for Origin and Diagenesis of Sparry Calcite Cement in Capitan Limestone, McKittrick Canyon, Permian Basin, West Texas: ABSTRACT

Sparry calcite cement is responsible for major porosity loss in the reef and fore-reef facies of the Capitan Limestone, Permian basin. This cement occurs throughout the massive reef facies where it fills large vugs as much as 0.5 m in diameter, with individual crystals up to several centimeters in length. Sparry cement in the fore-reef facies is less extensive and fills smaller vugs than in the reef, although individual crystals can be as large. The cement is composed of coarse, equant, clear to milky calcite that is rich in fluid inclusions. Homogenization temperatures (T/sub h/) of primary fluid inclusions trapped during cementation record formation temperatures from 55/sup 0/ - 85/sup 0/C. A relatively low geothermal gradient of 2/sup 0/C/100 m suggests cementation occurred at moderate burial depths of 1-2 km. Precipitation of sparry cement in fractures and brecciated areas also indicates cementation postdates minor burial compaction. Freezing temperatures (T/sub f/) for primary fluid inclusions range from about -1/sup 0/ to 0/sup 0/C (1.7 wt. % NaCl equivalent or less), suggesting the cement was formed in fresh to brackish waters. This range of T/sub f/ data is consistent with cement formation in a relatively deep lens of meteoric to mixing-zone waters.more » Secondary fluid inclusions with T/sub f/ of about -12/sup 0/C (> 16 wt. % NaCl equivalent) indicate that highly saline brines may have migrated along secondary fractures in the cement during later deposition of evaporites in the adjacent Castile and Salado Formations.« less

Thermal Transformation of Smectite to Illite: ABSTRACT

The diagenetic transformation of smectite to illite, with an intermediate series of mixed-layered compositions, has been documented by numerous studies of surface and subsurface rocks. Shales from typical Gulf Coast wells show compositions greater than 50% illite at approximately 100°C; the composition stabilizes at about 80% illite at the greatest depths sampled where the temperature is or has been as high as 130°C. These findings are difficult to reconcile with data from oil (lower Paleozoic) rocks if it is assumed that illite-smectite compositions are equilibrium phases at specific temperatures. Potash bentonites (Ordovician) End_Page 300------------------------------ associated with thin cratonic sediments contain 80% illite in the mixed-layered species, and these rocks were probably never heated over 60°C. In addition, many shales and limestones of Devonian age and older contain almost pure illite despite histories of only moderate burial depths. Studies of shales near contacts with basaltic dikes show that compositions near 80% illite are associated with peak temperatures (calculated) of approximately 300°C, though the duration of the heating event was short (^approx1,000 yr). A synthesis of available information suggests that the smectite to illite transformation is kinetically controlled, and that a high-order rate law is required, despite laboratory synthesis results which fit a first-order kinetic scheme. These conclusions are not accepted by all investigators, and the details of the reaction stoichiometry, kinetic mechanisms, and the possible occurrence of equilibrium intermediate compositions have yet to be convincingly demonstrated. End_of_Article - Last_Page 301------------

Petrology and Diagenesis of Early Miocene Deep-Sea Fan Deposits near Point Arena, California: ABSTRACT

Early Miocene deep-sea fan deposits of the Skooner Gulch and Gallaway Formations near Point Arena, California, were examined to determine which factors influence or control diagenesis in fine-grained turbidite deposits. Well-preserved sedimentary structures (including complete Bouma Ta-e beds) are nearly continuously exposed in a vertical sequence along the coast. This sequence is interpreted to represent outer fan depositional lobes, lobe fringe, and basin plain deposits. Four representative stratigraphic sections were sampled and analyzed using the scanning electron microscope, energy dispersive X-ray, X-ray diffraction, nuclear magnetic resonance, electron microprobe, and thin section analysis. Rock types examined include medium to very fine-grained arkoses, lithic arkoses, mudstones, siltstones, and hemipelagites. Diagenesis of a turbidite sequence is complex and does not involve uniform changes in detrital and authigenic minerals. Factors affecting diagenesis include variations in the original depositional environment, sediment composition, texture, stratification, frequency of interbedded shale beds, bioturbation, degree of compaction, and original porosity and permeability. Diagenetic changes recognized were: (1) alteration and resorption of unstable detrital grains, such as feldspar, rock fragments and detrital clay; (2) formation of authigenic quartz, potassium feldspar, and clay cements which reduced porosity and permeability; (3) formation of iron oxide and pyrite; and (4) incorporation of detrital and authigenic minerals in late-stage carbonate cement. All of the observed diagenetic cha ges can occur at relatively low temperatures and shallow to moderate burial depths. End_of_Article - Last_Page 1007------------

Carbonate Diagenesis in a Quartzarenite: Dolomitic Keefer Sandstone of West Virginia: ABSTRACT

After deposition as a coastal marine deposit, the Silurian Keefer Sandstone underwent an elaborate diagenetic sequence. In general, the processes were: quartz cementation, calcite cementation, dolomitization, and finally dedolomitization. Diagenesis in this formation, as interpreted from petrographic study, was complex because the composition of pore fluids changed throughout its postdepositional history. Quartz overgrowths precipitated early, most likely from marine connate brines. The second generation of cement, poikilotopic calcite, precipitated when meteoric-derived phreatic water percolated through the sand during a major regression of the sea. Clay minerals in the matrix favored dolomitization of calcite cement, fossils, and calcite mud by serving as sites of nucle tion or catalysts and perhaps by providing magnesium. Later, at a moderate burial depth, saline subsurface brines dedolomitized isolated dolomite crystals. Primary porosity in the Keefer (Big Six) of southwestern West Virginia was virtually occluded by these early cements. Most microscopically visible porosity is secondary and attributed to the volume reduction of carbonate material with dolomitization of local calcite. Several Silurian-Devonian sandstones are petrographically similar to the Keefer, and its postdepositional history, as interpreted here, may serve as an example of diagenesis in other dolomitic and calcareous quartzarenites of the Appalachian basin. End_of_Article - Last_Page 1289------------