Chemostratigraphic insights into fluvio-lacustrine deposition, Yanchang Formation, Upper Triassic, Ordos Basin, China

2017 ◽  
Vol 5 (2) ◽  
pp. SF149-SF165 ◽  
Author(s):  
Harry Rowe ◽  
Xiangzeng Wang ◽  
Bojiang Fan ◽  
Tongwei Zhang ◽  
Stephen C. Ruppel ◽  
...  
Keyword(s):  
Ordos Basin ◽  
Clay Content ◽  
Hierarchical Cluster ◽  
Enrichment Factors ◽  
Mineral Contents ◽  
Yanchang Formation ◽  
Fine Grained ◽  
Reducing Conditions ◽  
Surface Mixing ◽  
Lacustrine Facies

A chemostratigraphic study of a 300 m long core recovered from the southeastern central Ordos depocenter reveals thick ([Formula: see text]) intervals of fine-grained, organic-rich lacustrine strata, interpreted to represent deepwater deposition under meromictic conditions (incomplete watermass overturn) during lake highstand phases, interspersed with thick (10–30 m) intervals of arkosic sandstones, reflective of fluvio-deltaic deposition during lake lowstand phases. Along with elevated concentrations of %Al, traditionally a proxy for clay content, maximum total-organic-carbon (TOC) values in the deepwater lacustrine facies reach 8%, with average values of approximately 3%. The fine-grained, organic-rich facies is also characterized by elevated S (up to 6%) and As concentrations, both proxies for pyrite, an indicator of more stagnant, reducing conditions in the hypolimnion (the volume of the lake below the surface mixing zone) during lake highstand phases. Enrichment factors for redox-sensitive trace elements (RSTEs) are not significantly elevated throughout the TOC-rich intervals, but they are punctuated in thinner intervals. Punctuated enrichments in RSTEs reflect episodes of enhanced suboxia/anoxia in the hypolimnion during lake highstand phases resulting from sustained meromixis (protracted episodes of incomplete watermass turnover). Although dramatic shifts between fluvio-deltaic and deepwater lacustrine deposition are recorded in the Yanchang strata, no evidence indicates that the lacustrine system ever built up a significant salt concentration, suggesting that an outlet was maintained throughout much of the depositional history. A chemofacies framework for the Yanchang Formation is developed based on hierarchical cluster analysis and ranking of major element chemostratigraphic results. The outcome of chemofacies analysis is similar to the lithofacies analysis, subdividing the stratigraphic record of calcite-cemented sandstone/siltstone lithologies (fluvio-deltaic facies) and silty to finer grained mudstone lithologies (deep-water lacustrine facies) based on changes in elemental concentrations that equate to shifts in the relative abundance of mineral contents (e.g., clays, quartz, feldspars, pyrite, and carbonates).

scholarly journals Reservoir characteristics of Chang 2 Member of Yanchang Formation in Area A, Ordos Basin

2021 ◽  
Vol 290 ◽  
pp. 03011
Author(s):  
Yajie Jing ◽  
Zhiwu Zhang ◽  
Peng Xu ◽  
Shasha Yang
Keyword(s):  
Ordos Basin ◽  
Low Permeability ◽  
Pore Characteristics ◽  
Yanchang Formation ◽  
Reservoir Characteristics ◽  
Fine Grained ◽  
The Ordos Basin ◽  
Seepage Properties ◽  
Low Permeability Reservoirs ◽  
Good Medium

With the continuous enhancement of exploration and development in the Ordos Basin, in-depth research has been carried out on the petrological and reservoir characteristics of Chang 2 reservoir in Area A, which provides a geological basis for the efficient development of oil reservoirs. Comprehensive use of reservoir sandstone thin section identification casting analysis, mercury intrusion analysis, logging analysis and other methods to systematically study the petrological characteristics, pore characteristics and reservoir physical properties of Chang 2 reservoir in Area A. The results show that the reservoirs in the study area are dominated by fine-grained sandstones, with low component maturity and high structural maturity. They are all medium-low porosity, low-permeability and ultra-low permeability reservoirs. Primary intergranular pores and residual intergranular pores are developed, the reservoir drainage pressure is low, which is good-medium, and the mercury removal efficiency is high, indicating that the reservoir has good storage performance and seepage properties.

scholarly journals A new method of lithologic identification and distribution characteristics of fine - grained sediments: A case study in southwest of Ordos Basin, China

2019 ◽  
Vol 11 (1) ◽  
pp. 17-28
Author(s):  
Qiqi Lyu ◽  
Shunshe Luo ◽  
Yulong Guan ◽  
Jinhua Fu ◽  
Xiaobing Niu ◽  
...  
Keyword(s):  
Sedimentary Environment ◽  
Ordos Basin ◽  
Sedimentary Rocks ◽  
New Method ◽  
Lake Basin ◽  
Multivariate Linear Regression Analysis ◽  
Distribution Characteristics ◽  
Yanchang Formation ◽  
Fine Grained ◽  
Natural Gamma

Abstract Lithologic identification is critical for studying fine - grained sediments, which further elucidates sedimentary environment, and formation. The oil - bearing Chang 7 Section of the Yanchang Formation in Ordos Basin contains thick dark mudstone with a wide distribution, interbedded by siltstone and fine sandstone. The lacustrine fine - grained sedimentary rocks constitute the chief source rock of the Yanchang Formation. On the grounds of fine core description, thin rock slice identification and X - ray diffraction analysis, we proposed a new method based on conventional logging data. This method is using density (DEN) and natural gamma (GR) logging curve intersection and multivariate linear regression analysis of logging curve value and measured mineral content value which is carried out by SPSS software to identify the lithology and the vertical distribution characteristics of fine - grained sedimentary rock of Chang 7 formation in the study area. This method is mainly suitable for lithologic identification of fine - grained sedimentary rocks in lake basin. It not only quantitatively analyses the contents of main minerals and organic matter in fine - grained sedimentary rocks, but also greatly improved the accuracy and universality of using conventional logging data to identify the lithology of fine - grained sedimentary rocks, which provides a reference for the exploration of tight oil.

scholarly journals Development Characteristics and Finite Element Simulation of Fractures in Tight Oil Sandstone Reservoirs of Yanchang Formation in Western Ordos Basin

2022 ◽  
Vol 9 ◽  
Author(s):  
Shilin Wang ◽  
Hu Li ◽  
Lifei Lin ◽  
Shuai Yin
Keyword(s):  
Finite Element Method ◽  
Finite Element ◽  
Ordos Basin ◽  
Tight Oil ◽  
Yanchang Formation ◽  
Fine Grained ◽  
Low Amplitude ◽  
Distributary Channel ◽  
Sweet Spots ◽  
Element Method

Structural fractures have a significant control effect on the large-scale accumulation of hydrocarbons in the Yanchang Formation. Previous studies have affirmed the important role of fractures in hydrocarbon accumulations in strongly deformed zones. However, for low-amplitude structural areas, the degree of fracture development is relatively low, and their control on sweet spots of hydrocarbons has not yet formed a unified understanding. In this paper, taking the Upper Triassic Yanchang Formation in the western Ordos Basin as an example, the development characteristics, prediction method, and the distribution of fractures in tight sandstone reservoirs in low-amplitude structural areas have been systematically studied using a large number of cores, thin sections, paleomagnetism, FMI logging, acoustic emission, productivity data, and finite element method. The research results showed that the Yanchang Formation in the study area mainly develop high-angle and vertical fractures, which were formed by regional tectonic shearing. Fractures are mainly developed in the fine-grained and ultra-fine-grained sandstones of the distributary channel and estuary bar microfacies, while the fractures in the medium-grained sandstones of the distributary channel and the mudstones of the distributary bay are relatively underdeveloped. The core fractures and micro-fractures of the Yanchang Formation all have the regional distribution characteristics, and the fracture strikes are mainly between NE50° and NE 70°. Moreover, the finite element method was used to predict the fractures in the target layer, and the prediction results are consistent with the actual distribution results of the fractures. The coupling analysis of fractures and tight oil sandstone distribution showed that the existence of fractures provided conditions for the accumulation of hydrocarbons in the Yanchang Formation. The confluence and turning areas of the river channels were repeatedly scoured by river water, and the rocks were brittle and easy to form fractures. The thickness of the fractured sandstone in these areas is usually greater than 0.4 m. Moderately developed fracture zones are prone to form hydrocarbon accumulation “sweet spots,” and the fracture indexes of these areas are usually distributed between 0.8 and 1.2. However, when the fracture index exceeds 1.2, over-developed fractures are unfavorable for the accumulation of hydrocarbons.

scholarly journals Fluid inclusions in buried quartz of the Yanchang sandstone in the Jiyuan area, Ordos Basin, China: Implications for basin evolution and petroleum accumulation

2020 ◽  
pp. 014459872097451
Author(s):  
Wenqi Jiang ◽  
Yunlong Zhang ◽  
Li Jiang
Keyword(s):  
Fluid Inclusions ◽  
Ordos Basin ◽  
Homogenization Temperature ◽  
Burial Depth ◽  
Hydrocarbon Generation ◽  
Rock Interaction ◽  
Yanchang Formation ◽  
Reflection Data ◽  
And Migration ◽  
Vitrinite Reflection

A fluid inclusion petrographic and microthermometric study was performed on the sandstones gathered from the Yanchang Formation, Jiyuan area of the Ordos Basin. Four types of fluid inclusions in quartz can be recognized based on the location they entrapped. The petrographic characteristics indicate that fluid inclusions in quartz overgrowth and quartz fissuring-I were trapped earlier than that in quartz fissuring-IIa and fissuring-IIb. The homogenization temperature values of the earlier fluid inclusions aggregate around 80 to 90°C; exclusively, it is slightly higher in Chang 6 member, which approaches 95°C. The later fluid inclusions demonstrate high homogenization temperatures, which range from 100 to 115°C, and the temperatures are slightly higher in Chang 9 member. The calculated salinities show differences between each member, including their regression characteristics with burial depth. Combining with the vitrinite reflection data, the sequence and parameters of fluid inclusions indicate that the thermal history of the Yanchang formation mostly relied on burial. Salinity changes were associated with fluid-rock interaction or fluid interruption. Hydrocarbon contained fluid inclusions imply that hydrocarbon generation and migration occurred in the Early Cretaceous. The occurrence of late fluid inclusions implied that quartz cement is a reservoir porosity-loose factor.

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