Lower Paleozoic Carbonate Research Articles

Karstification and Its Role in the Formation of Carbonate Reservoirs: A Case Study from the Ordovician Majiagou Formation in Jingbian, Ordos Basin, North China

The discovery of natural gas reserves in Lower Paleozoic carbonate formations has generated significant enthusiasm regarding the potential for oil and gas exploration in the deeper carbonate reservoirs of the Ordos Basin. Significant progress has been made in the field of natural gas exploration, especially within the carbonate reservoirs of the Ordovician Majiagou Formation. In this study, we conducted a comprehensive analysis, including microscopic examination of thin sections and the inspection of 60-m cores from five wells, to classify pore types and investigate various forms of karstification in the fifth member of the Majiagou Formation. Our findings have identified distinct pore types, including interparticle pores, separate-vuggy pores, and touching-vuggy pores, in this formation. The dominant forms of karstification in this area were identified as syngenetic karstification, weathering crust karstification, and burial karstification. Importantly, our investigation emphasizes the significant influence of factors such as sea level fluctuations, exposure duration, and the presence of acidic formation water in shaping the observed patterns of karstification. Syngenetic karstification and shallow-buried karstification have emerged as key stages in the formation of natural gas reservoirs. Moreover, our research emphasizes the importance of structural and composite fractures observed in the fifth member of the Majiagou Formation. These unfilled fractures serve as crucial microfracture networks, facilitating oil and gas migration and contributing to the overall development of reservoirs. The research results are of great significance for understanding the formation process of carbonate reservoirs in the Majiagou Formation, Ordos Basin.

Scandium Ore Occurrences in the Ancient Weathering Crust in the Nakyn Kimberlite Field of Yakutia

In the Malo-Botuobinsky, Sredne-Markhinsky, and Ygyattinsky diamond-bearing areas of the Western Yakutian kimberlite province, the prerequisites and signs of scandium deposits in ancient weathering crusts overlying Lower Paleozoic rocks and associated with zones of ancient reservoir and soil oxidation. According to X-ray fluorescence and ICP MS analyses, in the Nakyn kimberlite field of the Sredne-Markhinsky diamond-bearing region, promising ore occurrences and scandium concentration halos have been established in clay deposits of redeposited weathering crusts of the Dyakhtar Formation of Late Triassic–Early Jurassic age. They are confined to the erosional surface of Lower Paleozoic carbonate rocks and Middle Paleozoic traps, with monzonite porphyries and kimberlites cutting through them, covered on top by a sedimentary cover of Jurassic terrigenous deposits. Concentration anomalies of scandium in clays of the Dyakhtar Formation are localized mainly in deluvial clay deposits of paleowatersheds and gravitate to the intersections of tectonic disturbances. The depth of their occurrence does not exceed 100 m, which is quite favorable for the extraction of scandium by borehole in situ leaching. Concentrations of scandium were detected at one of the sites before 262 g/t in Late Triassic weathering crust, including material from a Middle Paleozoic gabbro dike, as well as overlying colluvial clays of the Dyakhtar formations with a maximum of 462 g/t.

Characteristics and Controlling Factors of Crystalline Dolostone Reservoirs of the Sanshanzi Formation, Central-Eastern Ordos Basin, Northwestern China

In recent years, Ordos Basin has been the largest petroleum field in China and represents significant potential for gas exploration in the Lower Paleozoic carbonate reservoirs. Taking the central-eastern Ordos Basin as an example, this work studies the reservoir characteristics, origin, and controlling factors of the crystalline dolomites of the Sanshanzi Formation by means of petrological and geochemical studies. The Sanshanzi Formation dolostones comprise three kinds: crystalline dolostone with grain-shadow (Rd1), very fine–fine crystalline dolostone (Rd2), and medium-coarse crystalline dolostone (Rd3). The pore spaces include intercrystalline pores and intercrystalline solution pores, residual intergranular pores, vugs, and fractures. The medium-coarse crystalline dolostone is the best reservoir. The average porosity is 4.61%, and the average permeability is 0.91 mD. The sedimentary environment in the upper Cambrian was a large area of tidal flats. Under the influence of penecontemporaneous meteoric water leaching, dissolution developed at the top of the dolomite flat–lagoon depositional cycle, with vugs developing in the very fine–fine crystalline dolostone. Penecontemporaneous dolomitization enhanced the compaction resistance and effectively preserved the pore space of the reservoir. In addition, recrystallization improved the crystalline texture, turning the intergranular pores into residual intergranular pores and intercrystalline micropores into intercrystalline pores. In this case, the very fine–fine crystalline dolostone was transformed into medium-coarse crystalline dolostone. The results of this study provide a case for the in-depth understanding of the development and preservation mechanism of ancient dolomite reservoirs and also point to the research pathways for oil and gas exploration in the Cambrian in the Ordos Basin in the future.

Ordovician reservoirs in Fuxian area: Gas accumulation patterns and their implications for the exploration of lower Paleozoic carbonates in the southern Ordos Basin

In recent years, the Fuxian area in the southeastern Ordos Basin has undergone significant exploration, with industrial gas flow tested in wells drilled into the Ordovician marine carbonates. Despite this, the gas accumulation patterns of this area are not fully understood, posing challenges for further exploration. Our analysis of geological conditions indicates that the Ordovician Majiagou Formation in this area hosts two gas plays: one found in weathering crusts and the other found in interior of the formation. We investigated various typical gas reservoirs in the area, focusing on differentiating the geological conditions and factors controlling gas accumulation in the weathering-crust and interior gas reservoirs. The results suggest three primary gas accumulation patterns in the Majiagou Formation in the Fuxian area: (1) upper gas accumulation in weathering crusts, present in the high parts of landforms such as residual paleo-hills or buried paleo-platform (Pattern I); (2) the stereoscopic pattern with gas accumulation in both weathering crusts and strata interior, arising in high parts of landforms such as residual paleo-hills or buried paleo-platforms (Pattern II); (3) lower gas accumulation in strata interior, occurring in the upper reaches and on both sides of paleo-trenches (Pattern III). This study will serve as a geological basis for future exploration deployment in the Fuxian area.

Geochemical characteristics and their implication of light hydrocarbons in natural gas from the Lower Paleozoic carbonate reservoirs in the Daniudi gas field, Ordos Basin

The genetic types and source of the Lower Paleozoic natural gas in the Ordos Basin in China are controversial, and the geochemical characteristics of light hydrocarbons of natural gas in the fifth member of the Ordovician Majiagou Formation (O1m5) are studied by taking the Daniudi gas field as an example, in order to reveal the gas origin and source. The C5−7 light hydrocarbons are dominated by iso-alkanes rather than the n-alkanes, and the methyl cyclohexane (MCH) content is mostly higher than the nC7 content in C7 light hydrocarbons. The K1 and K2 values range from 1.06 to 1.16 and from 0.33 to 0.81, respectively. The geochemical characteristics of light hydrocarbons indicate that the Lower Paleozoic natural gas in the Daniudi gas field is mainly coal-derived gas which has been generated by the humic source rocks in the Upper Carboniferous Taiyuan Formation (C3t), with insignificant contribution by the O1m sapropelic source rocks. The O1m5 gas pools were accumulated through downward and lateral migration in the free phase. The aromatic contents in C6-7 and nC7/MCH ratios are easily affected by the factors such as water solubilization, adsorption in migration, evaporation, and migration fractionation, therefore, secondary alteration needs to be considered when using these parameters to identify the gas origin.

Multiple fluid flow events and diversity of hydrothermal minerals in Neoproterozoic to lower Paleozoic carbonate reservoirs, Tarim Basin, NW China

• Two stages of hydrothermal events account for carbonate-hosted mineralization . • Quartz precipitated from silica-rich fluids from the Precambrian sandstone layers. • Saddle dolomite, fluorite and calcite formed in the Cambrian shale-derived fluids. • The same parent fluid could precipitate different minerals in differing host rocks. • Basal sandstones do not always act as lateral conduits for hydrothermal fluids . Contrasting mineral assemblages have been frequently found in Neoproterozoic-lower Paleozoic carbonate reservoirs along high-angle faults in the Tarim Basin. However, the reason for the change in mineral types over time is poorly constrained. Field and petrographic observations, fluid inclusion microthermometry, rare earth element (REE) and C, O and Sr isotope analyses are used to constrain the changes in water chemistry and fluid flow during the development of the mineral assemblages. During the Middle Devonian-early Carboniferous, the flux of hydrothermal fluids from Neoproterozoic sandstone layers, linked to the activity of NE–SW-trending strike-slip faults, resulted in extensive silicification. The silica-bearing fluids were hot (128.1–209.4 °C), saline (17.1–21 wt% NaCl eq.) and enriched in radiogenic strontium and showed no link to large-scale petroleum migration. During the Permian, a regional thermal anomaly induced the expulsion of hydrocarbon-bearing hydrothermal fluids from lower Cambrian shales, where Mg 2+ , 87 Sr and F − were derived via the transformation and leaching of clay minerals. A more significant in situ source of Mg 2+ was the dolostone interval, as the migrating fluids contributed to the precipitation of saddle dolomite with high T h values (82–176.4 °C) and salinities (15–21.4 wt% NaCl eq.), negative δ 18 O values and radiogenic Sr compositions. The succession of saddle dolomite followed by fluorite with similar T h values, salinities and 87 Sr/ 86 Sr ratios indicates a Mg 2+ deficit and increased dissolved calcium activity in the same parent fluid system, which facilitated the breakdown of MgF − and combination of Ca 2+ and F − . Subsequently, cooling and desalination of the hydrothermal fluids occurred, which was induced by the infiltration and mixing of meteoric water related to the compression and uplift of the north-western flank of the Tarim block. These processes resulted in the precipitation of milky white calcite (Cal 1) with similar oxygen and strontium isotope compositions but wider ranges of T h (82.1–154 °C) and salinity (3.4–16.9 wt% NaCl eq.) than those of the saddle dolomite and fluorite. The lateral flow of hydrothermal fluids through suitable layers, such as bed interfaces, permeable aquifers and karstic units, facilitated the formation of stratabound mineral deposits away from the feeding faults. The termination of hydrothermal mineralization was marked by the precipitation of brown calcite (Cal 2), whose meteoric origin has been proven by the combination of its reddish staining, oscillatory luminescent zonation and low δ 18 O values. This integrated study approach improves the understanding of the diversity of exotic mineral assemblages in carbonate hosts and the evolution and migration of hydrothermal fluids connected to late Paleozoic tectonism.

Identification of High-Quality Transverse Transport Layer Based on Adobe Photoshop Quantification (PSQ) of Reservoir Bitumen: A Case Study of the Lower Cambrian in Bachu-Keping Area, Tarim Basin, China

The Lower Paleozoic carbonate reservoir in the Tarim Basin is a hotspot area for deep oil and gas exploration in China. Although the Lower Cambrian of the Bachu Uplift has not encountered industrial oil flow, rich bitumen has been found there. As the most direct trace of petroleum migration, the effective identification of bitumen is the key to studying the hydrocarbon transportation path. In this study, the Adobe Photoshop quantification (PSQ) method is used to identify the bitumen content in the Xiaoerblak Formation in Well Shutan 1, and, combined with the bitumen characteristics of the Shihuiyao section, a high-quality petroleum transverse transport layer is determined. The results indicate the following: (1) In Well Shutan 1, bitumen is mainly concentrated in the middle and upper parts of the Xiaoerblak Formation with high porosity and high permeability. (2) The shale of the Yuertus Formation in the Shihuiyao section has low hydrocarbon generation potential. However, the overlying Xiaoerblak Formation has developed multistage bituminous veins and bitumen-encapsulated gravels, which is the result of multiple instances of horizontal hydrocarbon migration. (3) After combining the bitumen characteristics of Well Shutan 1 and the Shihuiyao section, it is confirmed that there are high-quality lateral transport conductors in the middle and upper parts of the Xiaoerblak Formation, and the Subsalt Cambrian dolomite reservoir has great exploration potential.

Characteristics and Formation Mechanism of the Lower Paleozoic Dolomite Reservoirs in the Dongying Depression, Bohai Bay Basin

The Lower Paleozoic carbonate strata experience multi-stage tectonic activity and post-depositional volcanic activity in the Dongying Depression, Bohai Bay basin. These tectonic and magmatic activities have caused the reservoir to undergo severe diagenesis, resulting in strong reservoir heterogeneity. This study aims to identify the characteristics of dolomite, various reservoir spaces’ characteristics, the origin of different types of dolomite, and the porosity evolution. According to crystal size and morphology, dolomites can be divided into three kinds of matrix dolomites and four kinds of dolomite cements. The petrology and geochemistry of the dolomite suggests that matrix dolomite is formed from seawater. The medium-to-coarse-crystalline dolomite cement (D3) has a higher 87Sr/86Sr ratio (0.7119 to 0.7129) and a higher homogenization temperature (>125 °C), suggesting that the fluid for the precipitation of D3 is a mixed fluid formed by hydrothermal fluid eroding the 87Sr-rich feldspar sandstone. The strikingly negative δ18O values (−23.7 to −25.7‰ VPDB) of saddle dolomite (D4) indicate that D4 precipitated from hydrothermal fluids and the Mg2+ source may be due to dissolution of the host dolomite that formed in the evaporation environment. The reservoir spaces of the target strata in the study area mainly include fractures, dissolution vugs, intercrystalline pores, and moldic pores. Dissolution is the basis for forming high-quality dolomite reservoirs. The faults and fractures provided favorable conditions for dissolution. Hydrothermal fluid and organic acid were the main dissolution fluids for the dolomite reservoir, which were beneficial to the development of secondary pores. In the study area, organic acid dissolution was shown to contribute more than hydrothermal dissolution in the study area.

The breakthrough pressure and sealing property of Lower Paleozoic carbonate rocks in the Gucheng area of the Tarim Basin

Deep carbonate reservoirs with great resource potential are key areas for future hydrocarbon exploration with the continuous innovation of deep hydrocarbon accumulation theory and technology. Breakthrough pressure (BP) is the most important parameter used to indicate rock sealing. In this study, X-ray diffraction, CO2 adsorption, N2 adsorption, high-pressure mercury injection, thin section analysis, effective porosity-permeability and BP experiments are conducted to research pore-throat characteristic of carbonate rock and the influencing factor of BP. Pore volume increases with the specific surface area (SSA) of micropore, and has a weak negative and positive correlation with the SSA of mesopore and macropore, respectively. The BP increases with an increase in the micropore proportion and decreases with an increase in the macropore proportion. Compared with R50 (the pore-throat radii corresponding to these mercury saturation of 50%), R35 (the pore-throat radii corresponding to these mercury saturation of 35%) has a better correlation with porosity and permeability and can better characterize the influence of pore throats on the sealing performance of carbonate rock. A low clay content perhaps leads to an inapparent interrelation between clay content and BP. Because of high brittleness of quartz, an increase in quartz content will cause an increase of porosity and permeability, and a decrease in BP. The BP decreases with the increase in effective porosity and permeability, and linear correlations exist among the reciprocal of porosity (A), the −0.407 power of permeability (B), and the BP. The BP calculation formula is fitted based on A and B by the principal component regression (PCR) method of SAS software for carbonate rock in the Ordovician of the Gucheng area.

Porosity Modeling of Huge Thick Carbonates: A Case Study of Lower Paleozoic Carbonates in the Tazhong Uplift, Tarim Basin, China

This study provides a solution to determine the porosity evolution of huge thick carbonates accurately using PetroMod version 2016.2 software with a cementation tool. The Lower Paleozoic carbonates from the Tazhong Uplift (Tarim Basin) were taken as examples. Results show that the errors of modeled porosity decreased from 256.3%–468.8% to 6.7%–12.5% for low-porosity carbonates and from 15.0%–22.9% to 2.9%–5.0% for high-porosity carbonates, indicating a better fit to the measured values after considering carbonate cementation. The calibrated model incorporating cementation exhibits typically low carbonate porosities and burial depths but high thermal conductivities, resulting in much lower formation temperatures, maturities, and hydrocarbon masses than the uncalibrated model. This study indicated that carbonate cementation plays a very important role in the reduction of carbonate porosity, and it is necessary to calibrate the thick-carbonate porosity model using the cementation tool. Although this method has certain feasibility and practicability and can be utilized widely to model huge thick carbonates, it has a limitation in recovering the increase in porosity caused by postdiagenesis.

Structure-guided gravity inversion for layered density modeling with an application in the Chezhen Depression, Bohai Bay Basin

Gravity inversion, as a static potential field inversion, has inherent ambiguity with low vertical resolution. To reduce the nonuniqueness of the inversion, it is necessary to impose the a priori constraints derived by other geophysical inversions, drilling, or geologic modeling. Based on the a priori normalized gradients derived from seismic imaging or reference models, a structure-guided gravity inversion method with a few known point constraints is developed for mapping density with multiple layers. The cubic B-spline interpolation is used to parameterize the forward modeling calculation of the gravity response to smooth density fields. A recently adopted summative gradient is used to maximize the structural similarity between the a priori and inverted models. First, we have determined the methodology, followed by a synthetic fault model example to confirm its validity. Monte Carlo tests and uncertainty tests further examine the stability and practicality of the method. This method is easy to implement; consequently, it produces an interpretable density model with geologic consistency. Finally, we apply this method to the density modeling of the Chezhen Depression in the Bohai Bay Basin. Our work determines the distribution of deep Lower Paleozoic carbonate rocks and Archean buried hills with high-density characteristics. Our results are consistent with the existing formation mechanism of the “upper source-lower reservoir” type oil-gas targets.

Tectonic evolution analysis constrained jointly by in-situ calcite U-Pb dating and apatite fission track for southwestern Ordos Basin

The lower Paleozoic strata of Ordos Basin is rich in oil and gas resources. The Lower Paleozoic marine carbonate in the southwestern Ordos Basin is characterized by large thickness and high abundance of organic matter. The source rocks are in dry gas window.However, the tectonic evolution has not been well evaluated. The timing of the earlier tectonic events is a key scientific problem and needs to be addressed timely.In this study, under the constraints of detailed field investigations, microstructure characteristics, in-situ trace element analysis and regional deposition-burial evolution history, the secondary calcite in-situ U-Pb dating is combined with apatite fission-track time-temperature simulation curves of the Lower Paleozoic carbonate strata and its fault facture zone to reveal four tectonic events including the Late Ordovician, Late Triassic, Late Jurassic and end of the Early Cretaceous-Cenozoic.A hydrothermal event occurred at the Late Ordovician 452±13 Ma, which might be linked to the subduction and closure of the Proto-Tethys Ocean during the Caledonian period.A limited Indosinian tectono-thermal event occurred at 214±22 Ma, which might be caused by the regional fault movement.The intense tectonic activities during the Yanshanian and Himalayan periods induced a strong uplift and denudation.The southwestern Ordos Basin experienced multi-stage complex tectonic evolution and the caprock was seriously damaged, leaving rather unfavorable preservation conditions for hydrocarbon.This study conducts a novel tectonic evolution analysis by combining low-temperature thermochronology with in-situ U-Pb dating of fault-related calcite, providing a new way to analyze tectonic evolution and later transformation process at the margin of complex superimposed basins.

Variability and Main Controlling Factors of Hydrocarbon Migration and Accumulation in the Lower Paleozoic Carbonate Rocks of the Tazhong Uplift, the Tarim Basin, Northwest China

Hydrocarbon migration patterns and pathways were studied on the basis of three-dimensional seismic interpretation, drilling, geochemistry, production performance, and other data. Using these findings, the main factors controlling hydrocarbon migration and accumulation in the Lower Paleozoic carbonate rocks of the Tazhong Uplift were discussed. The spatiotemporal relationship between the hydrocarbon kitchens and pathway systems of the Tazhong Uplift and the spatial pattern of pathway systems were considered the main factors causing differences in hydrocarbon enrichment. Results also revealed that the Lower Paleozoic carbonates of the Tazhong Uplift have two hydrocarbon accumulation systems (inside and outside the source rocks). For the accumulation system within the source rocks, hydrocarbon migration and enrichment are vertically differentiated. Middle Cambrian gypsum salt rocks serve as the boundary, above which thrust and strike-slip faults mainly allow vertical transport of hydrocarbons. A multistage superposition pattern of strike-slip faults controls the differences in hydrocarbon enrichment on the periphery of the fault zone. Beneath the gypsum-salt rocks, hydrocarbon migration and enrichment is controlled by the topography of paleostructures and paleogeomorphology. For the hydrocarbon accumulation system outside the source rocks, hydrocarbon migration and enrichment are restricted by the layered pathway system, and the topography of the paleostructures and paleogeomorphology is the key factor controlling hydrocarbon enrichment. The Tazhong No. 1 Fault is the main vertical pathway system in the area underlain by no source rocks, and hydrocarbons are enriched at the periphery of the Middle-Lower Cambrian and No. 1 Fault Zone.

Geochemical characteristics of carbonates and indicative significance of the sedimentary environment based on carbon-oxygen isotopes, trace elements and rare earth elements: case study of the Lower Paleozoic carbonates in the Gucheng area, Tarim Basin, China

Geochemical characteristics of carbonates and indicative significance of the sedimentary environment based on carbon-oxygen isotopes, trace elements and rare earth elements: case study of the Lower Paleozoic carbonates in the Gucheng area, Tarim Basin, China

Reservoir forming conditions and exploration potential of Lower Paleozoic carbonate rocks in Gucheng area, Tarim Basin

The Gucheng region is oil and gas accumulation area of Tarim Basin，dominated by beach controlled lithologic oil and gas reservoirs，which has superior oil and gas accumulation conditions and broad exploration prospects. The lower Paleozoic strata in the Gucheng region are the Carboniferous, Ordovician, and Cambrian systems from top to bottom. The Silurian, Devonian, and Permian systems are missing. The lower Ordovician – Cambrian are the main exploration target layers in the area. There are four reservoirs: the Ordovician group, the Yingshan group, the Penglaiba group and the Cambrian group in The Gucheng region, The upper part of the Yijianfang formation and Yingshan formation are limestone reservoir, which is a set of potential reservoir. The lower part of the Yingshan formation, the Penglaiba formation, and the Cambrian formation developed three sets of dolomite reservoirs. The distribution of oil and gas in Gucheng region was mainly found in Yijianfang formation, Yingshan formation, Penglaiba formation and Cambrian of the Ordovician. The Yingshan group and the Hanwu group were the main exploration target layers. Yingshan formation and Cambrian are the main exploration targets, and Yijianfang formation and Penglaiba formation of Ordovician can be used as the concurrent exploration targets. Gucheng region can form two major realistic exploration areas: dolomite reservoir of Ordovician and reef beach of Cambrian. The Ordovician dolomite reservoirs are mainly searched for the inner shoal of the platform edge high-energy facies belt, and the Cambrian system is mainly search for the high-energy reef beach far away from the platform edge facies belt in the north of cherchen fault.

Quantifying shortening across the central Appalachian fold-thrust belt, Virginia and West Virginia, USA: Reconciling grain-, outcrop-, and map-scale shortening

AbstractWe present a kinematic model for the evolution of the central Appalachian fold-thrust belt (eastern United States) along a transect through the western flank of the Pennsylvania salient. New map and strain data are used to construct a balanced geologic cross section spanning 274 km from the western Great Valley of Virginia northwest across the Burning Spring anticline to the undeformed foreland of the Appalachian Plateau of West Virginia. Forty (40) oriented samples and measurements of >300 joint orientations were collected from the Appalachian Plateau and Valley and Ridge province for grain-scale bulk finite strain analysis and paleo-stress reconstruction, respectively. The central Appalachian fold-thrust belt is characterized by a passive-roof duplex, and as such, the total shortening accommodated by the sequence above the roof thrust must equal the shortening accommodated within duplexes. Earlier attempts at balancing geologic cross sections through the central Appalachians have relied upon unquantified layer-parallel shortening (LPS) to reconcile the discrepancy in restored line lengths of the imbricated carbonate sequence and mainly folded cover strata. Independent measurement of grain-scale bulk finite strain on 40 oriented samples obtained along the transect yield a transect-wide average of 10% LPS with province-wide mean values of 12% and 9% LPS for the Appalachian Plateau and Valley and Ridge, respectively. These values are used to evaluate a balanced cross section, which shows a total shortening of 56 km (18%). Measured magnitudes of LPS are highly variable, as high as 17% in the Valley and Ridge and 23% on the Appalachian Plateau. In the Valley and Ridge province, the structures that accommodate shortening vary through the stratigraphic package. In the lower Paleozoic carbonate sequences, shortening is accommodated by fault repetition (duplexing) of stratigraphic layers. In the interval between the duplex (which repeats Cambrian through Upper Ordovician strata) and Middle Devonian and younger (Permian) strata that shortened through folding and LPS, there is a zone that is both folded and faulted. Across the Appalachian Plateau, slip is transferred from the Valley and Ridge passive-roof duplex to the Appalachian Plateau along the Wills Mountain thrust. This shortening is accommodated through faulting of Upper Ordovician to Lower Devonian strata and LPS and folding within the overlying Middle Devonian through Permian rocks. The significant difference between LPS strain (10%–12%) and cross section shortening estimates (18% shortening) highlights that shortening from major subsurface faults within the central Appalachians of West Virginia is not easily linked to shortening in surface folds. Depending on length scale over which the variability in LPS can be applied, LPS can accommodate 50% to 90% of the observed shortening; other mechanisms, such as outcrop-scale shortening, are required to balance the proposed model.

Hydrothermal dolomite reservoirs in a fault system and the factors controlling reservoir formation-A case study of Lower Paleozoic carbonate reservoirs in the Gucheng area, Tarim Basin

The widespread hydrothermal activity in the Lower Paleozoic carbonate reservoirs of the Tarim Basin has received much attention. The discovery of hydrothermal dolomite reservoirs indicates that hydrothermal alteration may have a positive effect on reservoir development. However, the factors controlling hydrothermal dolomite reservoirs and their pore development characteristics are difficult to recognize and interpret. Based on the lithology, petrology, pore fabrics and geochemical characteristics of the Lower Paleozoic carbonate reservoirs in the Gucheng area, this research discloses that the origin of the carbonate reservoirs is related to alteration by hydrothermal brine. Further, hydrothermal dolomite reservoirs are strongly heterogeneous in different fault zone structures. This research divides the fault structures into fault cores (type 1) and damage zones (types 2 and 3). Type 1 is characterized by many breccias cemented by saddle dolomite, locally developed vugs or fractures in the cores, low density (1.7–2.8 g/cm3) and resistance (22.4–1075 Ω) in the log response, and shows abundant black-spotted vugs and sinusoidal fractures in formation microscanner images (FMI). Due to poor connectivity and high heterogeneity (permeability: 3.24 to 0.01 mD, average 0.59 mD; porosity: 5.1 to 0.7%, average 1.8%), type 1 hydrothermal dolomite reservoirs show both positive and negative hydrothermal fluid contributions. In the damage zones, especially in the areas near fault cores, type 2 is rich in fracture-pore systems in both the cores and FMI and is characterized by serrated fluctuations in acoustic waves and densities (2.4–2.8 g/cm3). Type2 performs well in terms of physical properties (permeability: 8.06 to 0.011 mD, average 0.837 mD; porosity: 6.6 to 0.65%, average 2.5%), indicating potentially favorable areas for the development of hydrothermal dolomite reservoirs. At the distal end of a damage zone or in poorly developed areas, type 3 features filling by hydrothermal products or higher densification (permeability: 0.77 to 0.0003 mD, average 0.125 mD; porosity: 1.6 to 0.2%, average 0.2%). The well logging response of type3, which has a density of around 2.8 g/cm3 and flat acoustic wave curves, is similar to that of the host rock. The differences in the development of hydrothermal dolomite reservoirs among diverse fault zones suggest that the damage zone (type 2) may be a potential area for hydrothermal dolomite reservoirs in the fault system. This study illustrates a model in which the hydrothermal fluid dissolves host rock in the proximal area and precipitates it at the distal end of the damage zone. The hydrothermal dolomite reservoir characteristics of different fault zones are also further interpreted and described. Therefore, different locations within a fault zone need to be considered and distinguished in studies of the factors controlling hydrothermal dolomite reservoirs in fault systems. This view may provide new ideas regarding whether hydrothermal activity in the Tarim Basin plays a positive role in the formation of reservoirs and offers new suggestions for the exploration of hydrothermal dolomite reservoirs in fault systems.

Modeling hydrocarbon accumulation based on gas origin and source rock distribution in Paleozoic strata of the Ordos Basin, China

Paleozoic gases in the Ordos Basin, China, are produced from upper Paleozoic tight sandstones, Ordovician weathered-crust, and ‘pre-salt’ (beneath gypsum deposits) dolomite reservoirs. Their origins have been debated, particularly as to whether Carboniferous–Permian (CP) or Ordovician source rocks contributed more gas to the lower Paleozoic gas reservoirs. The expanded exploration of lower Paleozoic gas to the eastern, western, northern, and southern regions of the basin has re-initiated the debates. In this study, geochemical characteristics of gas samples from newly drilled wells in different regions of Ordos Basin in combination with the source rock evaluation were used to determine the gas origin. The carbon and hydrogen isotope of alkane indicate that gas mixing occurred in both upper Paleozoic and Ordovician weathered-crust gases, and that TSR occurred in pre-salt gases. A modified δ13C–1/Cn plot was suggested to be useful to determine the carbon isotopic patterns of gases generated from the different source rocks. The light hydrocarbon characteristic shows that oil-type gas mainly distributes in the eastern basin. A study of source-rock distributions also indicates that the most favorable Ordovician source rock is located in the eastern basin. Based on the distribution of CP coal measures, limestones and lower Paleozoic carbonate source rock, a gas accumulation history was established. The results of this study provide applicable method for identifying gas origin in Ordos Basin. Moreover, the accumulation history proposed in this study is useful for further exploration of the lower Paleozoic gas in Ordos Basin.

Petroleum geological conditions and exploration potential of Lower Paleozoic carbonate rocks in Gucheng Area, Tarim Basin, China

Abstract Lower Paleozoic carbonate rocks are an important exploration area in craton area of the Tarim Basin, with the proven oil and gas reserves of more than 2.2×108 t, but no large-scale discovery has been made in the Gucheng area so far. The key issues restricting exploration are that the source rock, reservoir scale and law of oil and gas enrichment are unclear. By systematically examining the petroleum geological conditions of Lower Paleozoic carbonate rocks, the following findings are reached: (1) Source rocks of slope-basin facies developed in Cambrian-Lower Ordovician in the Gucheng area. (2) The dolomitized beach in the lower part of Ordovician Yingshan Formation has large-scale reservoirs, good reservoir-cap assemblage and developed gas source faults, and is an important field for increasing reserves and production in the near future; hydrocarbon enrichment is controlled by reservoir and gas source faults, and the central dolomitized beach zone is the main exploration area. (3) The Cambrian platform margin reef beach, large in scale, good in physical properties and close to source rocks, has the possibility to form monolithic gas field; the caprock and preservation conditions are the key factors for hydrocarbon enrichment; the northern part of the phases I and II platform margin reefs has better sealing conditions, and is the main direction of next exploration. (4) Limestone fault solution reservoirs in the upper part of Ordovician Yingshan Formation, controlled by faults and small in scale, but good in reservoir-cap combination, worth exploring. (5) The granular limestone beach of Ordovician Yijianfang Formation is well developed and gas-bearing, but short in exposure dissolution time, and the reservoirs are strongly heterogeneous, and are a potential exploration field.

An exploration breakthrough in Paleozoic petroleum system of Huanghua Depression in Dagang Oilfield and its significance, North China

Abstract In recent years, several wells in the Qibei and Wumaying buried hills of Dagang Oilfield tapped oil in the Carboniferous–Permian and Ordovician strata. This major breakthrough reveals that the deep Paleozoic in the Bohai Bay is a new petroleum system. Through re-evaluating the Paleozoic source rock, reservoir-cap combinations and traps, it is found the oil and gas mainly come from Carboniferous–Permian source rock. The study shows that the Paleozoic strata are well preserved in the central-south Huanghua Depression and developed two kinds of reservoirs, Upper Paleozoic clastic rock and Lower Paleozoic carbonate rock, which form favorable source-reservoir assemblages with Carboniferous–Permian coal measure source rock. The Carboniferous–Permian coal-bearing source rock is rich in organic matters, which are mainly composed of type II2 and III kerogens, and minor II1 kerogen in partial areas. Multi-stage tectonic movements resulted in two stages of hydrocarbon generation of the source rocks. The period from the deposition of Kongdian Formation to now is the second stage of hydrocarbon generation. The matching between large-scale oil and gas charging, favorable reservoir-cap combinations and stable structure determines the enrichment of oil and gas. According to the new comprehensive evaluation of Paleozoic petroleum system, the primary oil and gas resources of the Paleozoic in the Bohai Bay Basin are over 1×1012 m3. The exploration breakthrough in Paleozoic petroleum system, especially Carboniferous–Permian petroleum system in Huanghua Depression is inspirational for oil and gas exploration in similar provinces of Bohai Bay Basin.