Characteristics and Origin of Organic Matter in Triassic Lacustrine Shale From Fuxian Oilfield

Fan Bojiang

doi:10.3389/feart.2021.752954

Abstract

Along with the successful exploitation of shale oil, organic-rich shale has attracted great attention to the geologists. However, quantitatively differentiating the origin of organic matter in shale is still challenging due to the nature of its complex depositional processes. Organic matter in lacustrine shales could be derived from both lake water (produced by lake water column) and land input (transported into the lake by rivers). The Triassic Chang-7 shale in the Yanchang Formation is a typical lacustrine shale deposited in the central part of the Ordos Basin. It is rich in organic matter (TOC of 0.52−9.37%) and consists primarily of type II1 kerogen. The Chang-7 shale is thermally mature and currently in the stage of generating oil and gas, with more structured than organic matter. In this study, quantitative models of organic matter in the Chang-7 shale with sources from both lake water and land is established using geochemical and carbon isotopes data and the results show that lake water and land input have the almost equal contribution to the organic enrichment in the study area.

Highlights

Along with the successful exploitation of shale oil, organic-rich shale has attracted more and more attention to the petroleum geologists (Fink et al, 2018; Awan et al, 2020)
The samples with total organic carbon content (TOC) < 2.00% account for 48.00% of the total samples; the samples with TOC < 4.00% account for 32.00% of the total samples; the samples with TOC > 4.00% account for 20.00% of the total samples
The aquatic organic index has little variations in the study area, ranging from 0.75 to 0.99. This is probably due to the simple chemical nature and weak carbon isotope fractionation in the shallow lake where the Chang-7 shale was deposited (Figure 10)

Summary

INTRODUCTION

Along with the successful exploitation of shale oil, organic-rich shale has attracted more and more attention to the petroleum geologists (Fink et al, 2018; Awan et al, 2020). Based on the analysis of 38 samples in the study area, there is only a minor variation in the values of Pr/n-C17 and Ph/ n-C18, proving that the organic matter in Chang-7 shale has a mixed origin (Figure 9). The aquatic organic index has little variations in the study area, ranging from 0.75 to 0.99 (only two samples are less than 0.8) This is probably due to the simple chemical nature and weak carbon isotope fractionation in the shallow lake where the Chang-7 shale was deposited (Figure 10). The data and analysis mentioned above indicate that paleo-water column and terrestrial input contributed to the enrichment of organic matter in the Chang-7 shale in the study area. Maximum value of carbonate carbon isotopes and minimum value of carbonate carbon isotopes, are difficult to be obtained, which limits its feasibility to be applied in other areas or scenarios

CONCLUSION

Findings

DATA AVAILABILITY STATEMENT