Geochemical characteristics of the Chang7 organic-rich fine-grained sedimentary rocks and its relationship with the tight oil in Longdong area, Northwest China

Yongliang Gao,Yamin Yu,Wei Li,Yaohui Xu,Huanxin Song,Changwei Ke,Zhigang Wen

doi:10.1007/s13202-020-00891-8

Abstract

In order to study the geochemical characteristics of fine-grained sedimentary rocks and clarify its relationship with the tight oil reservoir, the Chang7 Member of Triassic Yanchang Formation in Longdong area was taken as an example to be studied by rock pyrolysis, isotopic determination, maceral analysis, gas chromatography and gas chromatography–mass spectrometry. Lacustrine organic-rich fine-grained sedimentary rocks can be divided into organic-rich shale and organic-rich mudstone according to their lithology and texture. The results show that the organic-rich shale was deposited in a quiet and anoxic deep lacustrine environment, the organic matter was of high abundance and mainly sourced from plankton and benthic algae (type I–II1). Organic-rich mudstone mainly deposited in the semi-deep lacustrine environment, where the water was relatively shallow and the salinity was low, the organic matter was of low abundance and came from the mixed source of plankton and terrestrial clastic (type II1–II2). The Chang7 tight oil is widely distributed in the turbidite sand bodies, and it was mainly sourced from the Chang7 organic-rich shale.

Highlights

This paper aims to summarize the geochemical characteristics of different types of lacustrine organic-rich finegrained sedimentary rocks in Ordos Basin and analyze their relationship with the accumulation of tight oil
Shale samples mainly distribute in the section of type I–II1, while mudstone samples mainly distribute in the section of type II1–II2
The organic petrological indexes that characterize the development degree of fine-grained sedimentary rocks are significantly influenced by the sedimentary environment and preservation conditions

Summary

Introduction

With the development of tight oil exploration and research in related fields, the relationship between fine-grained sedimentary rocks and tight oil accumulation has become a hot topic in current research (Jia et al 2012; Jiang et al 2013; Du et al 2014; Hao et al 2014; Ghanizadeh et al 2015; PuAccording to the difference of TOC, clay mineral and carbonate mineral content, fine-grained sedimentary rocks were divided into six categories: high organic shale clay stone, high organic shale limestone, medium organic shale clay stone, medium organic shale limestone, low organic shale clay stone, and low organic shale limestone (Jiang et al 2013). According to the particle size, sedimentary structure and geochemical characteristics, fine-grained sedimentary rocks of Chang Member in Ordos Basin were divided into 4 lithofacies: oil shale facies, dark mudstone facies, sandy mudstone and. Siltstone and argillaceous siltstone are developed in sandy lithofacies The structure of these rocks is relatively tight, and tight oil resources can be formed when oil and gas are accumulated in large quantities (Pommer et al 2015; Hu et al 2015; Nole et al 2016; Ko et al 2016; Lucy et al 2016; Zhang et al 2018). Organic-rich fine-grained sedimentary rocks are developed in oil shale facies and dark mudstone facies. It can be seen that organic-rich fine-grained sedimentary rocks, as high-quality source rocks, control the formation and distribution of conventional oil and gas reservoirs, and are closely related to unconventional oil and gas resources such as tight oil and gas

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Conclusion