Characteristics of Chang 7 shale gas in the Triassic Yanchang Formation, Ordos Basin, China

2017 ◽  
Vol 5 (2) ◽  
pp. SF31-SF39 ◽  
Author(s):  
Xiangzeng Wang
Keyword(s):  
Organic Matter ◽  
Clay Mineral ◽  
Shale Gas ◽  
Mineral Content ◽  
Ordos Basin ◽  
Central China ◽  
Late Triassic ◽  
Yanchang Formation ◽  
Geochemical Parameters ◽  
Porosity And Permeability

The Yanchang Formation in the Ordos Basin in North Central China represents a large, long-lived lacustrine system of the late Triassic Period. The extensive shales within this system provide hydrocarbons (HCs) for conventional and unconventional oil and gas reservoirs. In the formation, the Chang 7 shale is the thickest shale with the best geochemical parameters, and it is the main source rock in this area. In recent years, the discovery of shale gas in the Chang 7 shale has promoted the exploration and development of lacustrine shale gas in China. We have estimated the shale gas resource potential based on the analysis of the geologic conditions of the Chang 7 shale. The average thickness of the Chang 7 shale reaches 42.6 m, and the main organic matter types are types [Formula: see text] and [Formula: see text]. The average content of organic carbon is more than 3%, and the average HC potential is [Formula: see text]. However, the thermal maturity of the Chang 7 shale is low with a vitrinite reflectance [Formula: see text] ranging from 0.83% to 1.10%. The Chang 7 shale lithology consists of shale and sandy laminations or thin sandstones. The shale is characterized by high clay mineral content and poor porosity and permeability, with an average porosity of 1.8% and an average permeability of [Formula: see text]. The sandy laminations or thin sandstones are characterized by relatively higher brittle mineral content, relatively lower clay mineral content, and higher porosity and permeability. The pores of the Chang 7 shale include primary intergranular and intragranular pores, secondary intragranular and intragranular dissolved pores, fracture pores, and organic-matter-hosted pores. The proportion of adsorbed gas, free gas, and dissolved gas is approximately 52%, 37%, and 11%, respectively, and the shale gas resources of the Chang 7 shale are [Formula: see text].

Pore types, pore-network analysis, and pore quantification of the lacustrine shale-hydrocarbon system in the Late Triassic Yanchang Formation in the southeastern Ordos Basin, China

2017 ◽  
Vol 5 (2) ◽  
pp. SF63-SF79 ◽  
Author(s):  
Robert G. Loucks ◽  
Stephen C. Ruppel ◽  
Xiangzeng Wang ◽  
Lucy Ko ◽  
Sheng Peng ◽  
...  
Keyword(s):  
Organic Matter ◽  
Clay Mineral ◽  
Mineral Content ◽  
Ordos Basin ◽  
Pore Network ◽  
Black Shales ◽  
Late Triassic ◽  
Ductile Material ◽  
Crushed Rock ◽  
Porosity And Permeability

Continental Upper Triassic Yanchang “black shales” in the southeastern Ordos Basin have been proven to be unconventional gas reservoirs. Organic-matter-lean and organic-matter-rich argillaceous mudstones form reservoirs that were deposited in a deeper water lacustrine setting during lake highstands. In the stratified lake, the bottom waters were dysaerobic to anoxic. This low-energy and low-oxygen lake-bottom setting allowed types II and III organic matter to accumulate. Interbedded with the argillaceous mudstones are argillaceous arkosic siltstones deposited by gravity-flow processes. Rock samples from the Yanchang Chang 7–9 members are very immature mineralogically. Mineral grains are predominantly composed of relatively equal portions of quartz and feldspar. The high clay-mineral content, generally greater than 40%, has promoted extensive compaction of the sediments, permitting the ductile material to deform and occlude interparticle pores. Furthermore, this high clay-mineral content does not favor hydraulic fracturing of the mudstone reservoir. The pore network within the mudstones is dominated by intraparticle pores and a lesser abundance of organic-matter pores. Interparticle pores are rare. The mean Gas Research Institute (GRI) crushed-rock porosity is 4.2%. Because the pore network is dominated by poorly connected intraparticle pores, permeability is very low (the GRI-calculated geometric mean permeability = 9.9 nd). The dominance of intraparticle pores creates a very poor correlation between GRI porosity and GRI permeability. Several methods of porosity analysis (GRI crushed rock, nitrogen adsorption, and point count) were conducted on each samples, and the results were compared. There is no significant correlation between the three methods, implying that each method measures different pore sizes or types. There is also no relationship between the porosity and permeability and total organic carbon. Much of the mature (peak oil window) organic matter is nonporous, suggesting that it is of type III. Most of the organic-matter pores are in migrated solid bitumen. Overall, the samples analyzed have low porosity and permeability for mudrocks.

Fractures in continental shale reservoirs: a case study of the Upper Triassic strata in the SE Ordos Basin, Central China

2015 ◽  
Vol 153 (4) ◽  
pp. 663-680 ◽  
Author(s):  
WENLONG DING ◽  
PENG DAI ◽  
DINGWEI ZHU ◽  
YEQIAN ZHANG ◽  
JIANHUA HE ◽  
...  
Keyword(s):  
Organic Matter ◽  
Shale Gas ◽  
Mineral Content ◽  
Ordos Basin ◽  
Total Content ◽  
Upper Triassic ◽  
Thin Sections ◽  
Yanchang Formation ◽  
Fracture Development ◽  
Shale Reservoirs

AbstractFractures are important for shale-gas reservoirs with low matrix porosity because they increase the effective reservoir space and migration pathways for shale gas, thus favouring an increased volume of free gas and the adsorption of gases in shale reservoirs, and they increase the specific surface area of gas-bearing shales which improves the adsorption capacity. We discuss the characteristics and dominant factors of fracture development in a continental organic matter-rich shale reservoir bed in the Yanchang Formation based on observations and descriptions of fracture systems in outcrops, drilling cores, cast-thin sections and polished sections of black shale from the Upper Triassic Yanchang Formation in the SE Ordos Basin; detailed characteristics and parameters of fractures; analyses and tests of corresponding fracture segment samples; and the identification of fracture segments with normal logging. The results indicate that the mineral composition of the continental organic-matter-rich shale in the Yanchang Formation is clearly characterized by a low brittle mineral content and high clay mineral content relative to marine shale in the United States and China and Mesozoic continental shale in other basins. The total content of brittle minerals, such as quartz and feldspar, is c. 41%, with quartz and feldspar accounting for 22% and 19% respectively, and mainly occurring as plagioclase with small amounts of carbonate rocks. The total content of clay minerals is high at up to 52%, and mainly occurs as a mixed layer of illite-smectite (I/S) which accounts for more than 58% of the total clay mineral content. The Upper Triassic Yanchang Formation developed two groups of fracture (joint) systems: a NW–SE-trending system and near-E–W-trending system. Multiple types of fractures are observed, and they are mainly horizontal bedding seams and low-dip-angle structural fractures. Micro-fractures are primarily observed in or along organic matter bands. Shale fractures were mainly formed during Late Jurassic – late Early Cretaceous time under superimposed stress caused by regional WNW–ESE-trending horizontal compressive stress and deep burial effects. The extent of fracture development was mainly influenced by multiple factors (tectonic factors and non-tectonic factors) such as the lithology, rock mechanical properties, organic matter abundance and brittle mineral composition and content. Specifically, higher sand content has been observed to correspond to more rapid lithological changes and more extensive fracture development. In addition, higher organic matter content has been observed to correspond to greater fracture development, and higher quartz, feldspar and mixed-layer I/S contents have been observed to correspond to more extensive micro-fracture development. These results are consistent with the measured mechanical properties of the shale and silty shale, the observations of fractures in cores and thin-sections from more than 20 shale-gas drilling wells, and the registered anomalies from gas logging.

Facies, rock attributes, stratigraphy, and depositional environments: Yanchang Formation, Central Ordos Basin, China

2017 ◽  
Vol 5 (2) ◽  
pp. SF15-SF29 ◽  
Author(s):  
Stephen C. Ruppel ◽  
Harry Rowe ◽  
Kitty Milliken ◽  
Chao Gao ◽  
Yongping Wan
Keyword(s):  
Organic Matter ◽  
Clay Mineral ◽  
Ordos Basin ◽  
Depositional Environments ◽  
Source Rocks ◽  
Late Triassic ◽  
Great Promise ◽  
Yanchang Formation ◽  
X Ray ◽  
Stacking Patterns

The Late Triassic Yanchang Formation (Fm) is a major target of drilling for hydrocarbons in the Ordos Basin. Although most of the early focus on this thick succession of lacustrine rocks has been the dominant deltaic sandstones and siltstones, which act as local reservoirs of oil and gas, more recent consideration has been given to the organic-rich mudstone source rocks. We used modern chemostratigraphic analysis to define vertical facies successions in two closely spaced cores through the Chang 7 Member, the primary source rock for the Yanchang hydrocarbon system. We used integrated high-resolution X-ray fluorescence and X-ray diffraction measurements to define four dominant facies. Variations in stable carbon isotopes mimic facies stacking patterns, suggesting that terrigenous organic matter (although minor in volume) is associated with the arkoses and sandstones, whereas aquatic organic matter is dominant in the mudstones. Facies stacking patterns define three major depositional cycles and parts of two others, each defined by basal mudstone facies that document basin flooding and deepening (i.e., flooding surfaces). Unconfined compressive strength measurements correlate with clay mineral abundance and organic matter. Comparisons of core attributes with wireline logs indicate that although general variations in clay mineral volumes (i.e., mudstone abundance) can be discerned from gamma-ray logs, organic-matter distribution is best defined with density or resistivity logs. These findings, especially those established between the core and log data, provide a powerful linkage between larger scale facies patterns and smaller scale studies of key reservoir attributes, such as pore systems, mineralogy, diagenesis, rock mechanics, hydrocarbon saturation, porosity and permeability, and flow parameters. This first application of modern chemostratigraphic techniques to the Yanchang Fm reveals the great promise of applying these methods to better understand the complex facies patterns that define this lacustrine basin and the variations in key reservoir properties that each facies displays.

scholarly journals Relationship between Organic Geochemistry and Reservoir Characteristics of the Wufeng-Longmaxi Formation Shale in Southeastern Chongqing, SW China

Energies ◽  
2021 ◽  
Vol 14 (20) ◽  
pp. 6716
Author(s):  
Shengxiu Wang ◽  
Jia Wang ◽  
Yuelei Zhang ◽  
Dahua Li ◽  
Weiwei Jiao ◽  
...  
Keyword(s):  
Organic Matter ◽  
Specific Surface ◽  
Adsorption Capacity ◽  
Surface Area ◽  
Clay Mineral ◽  
Shale Gas ◽  
Mineral Content ◽  
Organic Geochemistry ◽  
Quartz Content ◽  
Longmaxi Formation

Shale gas accumulates in reservoirs that have favorable characteristics and associated organic geochemistry. The Wufeng-Longmaxi formation of Well Yucan-6 in Southeast Chongqing, SW China was used as a representative example to analyze the organic geochemical and reservoir characteristics of various shale intervals. Total organic carbon (TOC), vitrinite reflectance (Ro), rock pyrolysis, scanning electron microscopy (SEM), and nitrogen adsorption analyses were conducted, and a vertical coupling variation law was established. Results showed the following: the Wufeng-Longmaxi formation shale contains kerogen types I and II2; the average TOC value at the bottom of the formation is 3.04% (and the average value overall is 0.78%); the average Ro value is 1.94%; the organic matter is in a post mature thermal evolutionary stage; the shale minerals are mainly quartz and clay; and the pores are mainly intergranular, intragranular dissolved pores, organic matter pores and micro fractures. In addition, the average specific surface area (BET) of the shale is 5.171 m2/g; micropores account for 4.46% of the total volume; the specific surface area reaches 14.6%; and mesopores and macropores are the main pore spaces. There is a positive correlation between TOC and the quartz content of Wufeng-Longmaxi shale, and porosity is positively correlated with the clay mineral content. It is known that organic pores and the specific area develop more favorably when the clay mineral content is higher because the adsorption capacity is enhanced. In addition, as shale with a high clay mineral content and high TOC content promotes the formation of a large number of nanopores, it has a strong adsorption capacity. Therefore, the most favorable interval for shale gas exploration and development in this well is the shale that has a high TOC content, high clay mineral content, and a suitable quartz content. The findings of this study can help to better identify shale reservoirs and predict the sweet point in shale gas exploration and development.

scholarly journals Geochemistry and shale gas potential of the lower Permian marine-continental transitional shales in the Eastern Ordos Basin

2020 ◽  
pp. 014459872097924
Author(s):  
Jingyi Wei ◽  
Yongli Wang ◽  
Gen Wang ◽  
Zhifu Wei ◽  
Wei He
Keyword(s):  
Organic Matter ◽  
Organic Carbon ◽  
Total Organic Carbon ◽  
Shale Gas ◽  
Higher Plants ◽  
Ordos Basin ◽  
Organic Matter Content ◽  
Lower Permian ◽  
Gas Desorption ◽  
Geochemical Parameters

Marine–continental transitional strata were widely developed in the Ordos Basin in Upper Carboniferous - Lower Permian. The Taiyuan - Shanxi Formation possesses promising shale gas exploration layers. Shale samples from two drilling wells of Shanxi-Taiyuan Formation in Shilou and Xixian, Ordos Basin, were investigated to study their carbon–sulfur contents and distribution characteristics of organic components using carbon/sulfur analyzer and gas chromatography–mass spectroscopy. Using results of total organic carbon analyses, Rock-Eval pyrolysis, X-ray diffraction analysis, shale gas desorption experiments, and other relevant experimental data, the shale samples were comprehensively analyzed. The exploitability of the shale in the study area was evaluated. The Shanxi-Taiyuan Shale in the Shilou and Xixian areas was characterized by high total organic carbon contents of 7.1% and 2.1% and high Tmax values of 499 and 505 °C, respectively. The organic matter of the shale is types II2 and III. Moreover, biomarker parameters including n-alkanes, Paq, Pwax, average carbon chain length, and the ternary diagram of C27-C28-C29 steranes show the organic matter constituted terrestrial higher plants and aquatic low biological algae. Multiple n-alkane parameters show the organic matter input in the Shilou area is mainly derived from terrestrial higher plants. The Pr/Ph value and trace element indicators show the deposition environment is dominated by weak oxidation–reduction conditions. A shale gas desorption experiment shows the average desorbed gas contents of the shale samples in the Shilou and Xixian areas were 1.79 and 0.37 m3/t, respectively. The organic matter content determined the differences in shale gas properties between the two areas in Ordos Basin. The composition and content of inorganic minerals affect the reservoir physical properties. According to the analyses, the shale in the Shilou area has good shale gas reservoir characteristics in terms of desorbed gas content and the above-mentioned geochemical parameters. Furthermore, the Shanxi shale has good potential for shale gas industrial exploitation.

scholarly journals Organic petrology of peak oil maturity Triassic Yanchang Formation lacustrine mudrocks, Ordos Basin, China

2017 ◽  
Vol 5 (2) ◽  
pp. SF211-SF223 ◽  
Author(s):  
Paul C. Hackley ◽  
Lixia Zhang ◽  
Tongwei Zhang
Keyword(s):  
Organic Matter ◽  
Ordos Basin ◽  
Central China ◽  
Type I ◽  
Peak Oil ◽  
Green River ◽  
Yanchang Formation ◽  
Solid Bitumen ◽  
Organic Petrology ◽  
Original Organic Matter

An organic petrology evaluation and a determination of solid bitumen reflectance [Formula: see text] were completed for organic-rich Triassic Yanchang Formation mudrocks ([Formula: see text]) from the Ordos Basin, north-central China, as part of a larger investigation of “shale gas” resources. These data were integrated with information from Rock-Eval programmed pyrolysis to show that the samples are in the peak oil window of thermal maturity and that organic matter is dominated by solid bitumen with minor amounts of type III kerogen (vitrinite and inertinite) from vascular land plants. Describing a “kerogen type” for these rocks based strictly on parameters determined from programmed pyrolysis is misleading because the original organic matter has converted to hydrocarbons (present as solid bitumen), a large proportion of which may have been expelled into adjacent reservoir facies. However, based on the comparison with immature-early mature lacustrine mudrock (Garden Gulch Member of Green River Formation) and marine shale (Boquillas Formation), we suggest that the original organic matter in the organic-rich samples examined for our study may have been type I/II kerogen with hydrogen index values of [Formula: see text] TOC.

Sign in / Sign up

Export Citation Format

Share Document