Geochemical and geological characterization of marine–continental transitional shales from Longtan Formation in Yangtze area, South China. (September 2018)

• Record Type: Journal Article
• Authors: Zhang, Jizhen; Li, Xianqing; Zhang, Xueqing; Zhang, Min; Cong, Gangshi; Zhang, Guangwu; Wang, Feiyu
• Is Part Of: Marine and petroleum geology. Volume 96(2018)
• Page Start: 1
• Page End: 15
• Publication Date: 2018-09
• Subjects: Organic geochemistry -- Marine-continental shale -- Longtan formation -- Microscopic pores -- Yangtze area -- Shale gas; EIA Energy Information Administration -- OM organic matter -- TOC total organic carbon -- Ro vitrinite reflectance -- Fm Formation -- GC–MS gas chromatograph–mass spectrometry -- XRD X–ray diffraction -- FE-SEM field emission scanning electron microscopy -- SKLPRP State Key Laboratory of Petroleum Resource and Prospecting -- S1 volatile hydrocarbon content -- S2 remaining hydrocarbon generative potential -- Tmax temperature of maximum pyrolysis yield -- PC effective carbon -- HIC hydrocarbon index -- HI hydrogen index -- PI production index -- FID flame ionization detector -- Pr pristane -- Ph phytane -- CPI carbon preference index -- OEP odd–even preference -- Tm trisnorhopane -- Ts trisnorneohopane -- I/S illite/smectite -- InterP interparticle -- IntraP intraparticle; Submarine geology -- Periodicals; Petroleum -- Geology -- Periodicals; Géologie sous-marine -- Périodiques; Pétrole -- Géologie -- Périodiques; Petroleum -- Geology; Submarine geology; Periodicals; Electronic journals; 551.468
• DOI: 10.1016/j.marpetgeo.2018.05.020
• Languages: English
• ISSNs: 0264-8172

Abstract

Abstract: Although previous research has provided useful insights into unconventional shale gas resources in recent years, the geochemical and geological characteristics of marine–continental transitional shale have not been studied systematically. During the Late Permian, Longtan Formation shales were widely deposited in the Yangtze area of South China. These shales clearly indicate the need to deepen our understanding of marine–continental transitional shales. This study describes the various characteristics of black shales from the Longtan Formation in this area on the basis of a field investigation and relevant laboratory analyses. Longtan shale reservoirs, with a thickness and burial depth range of 20–650 and 800–2400 m, respectively, are usually interbedded with coal and compact sandstone reservoirs due to their special sedimentary environment. Geochemical analysis of the samples of Longtan shales from seven wells in the Yangtze area indicates the presence of high organic matters (OMs), with a total organic carbon content in the range of 0.85%–35.7% (average of 7.33%) but low hydrocarbon genetic potential (S2 ) with a value below 3 mg HC/g rock. The analysis of carbon isotopes and organic macerals demonstrates that the investigated samples mainly contain type III kerogen. Data from gas chromatography and gas chromatography-mass spectrometry analyses illustrate a weakly oxidizing to a weakly reducing environment, with most organic materials originating from algal-bacterial organisms and terrestrial plants. The terrestrial plants contribute significantly to the OM. The Longtan shales are thermally over-matured, and they have entered the dry-gas generation window according to Ro, production index data, and biomarker maturity ratios. X-ray diffraction analyses show that the content of brittle minerals of the Longtan shales is generally lower than that of marine shales in North America. The Longtan shales show a high clay content averaging 60.32 wt % and low calcite mineral content averaging 3.73 wt %. Field emission scanning electron microscopy shows that various types of nanometer-to micrometer-scale pores, including interparticle and intraparticle pores, OM pores, and microfractures, are well developed in the shales. Generally, high OM abundance, thermal evolution degree, clay mineral content, and pore space (porosity of 0.56%–10.6%) positively influence shale gas content (1.0%–3.8%). However, high clay content and variable reservoir thickness hinder the successful production of shale gas. Highlights: Highly organic-rich, thermally over-matured and type III kerogen are developed. Mixed organic matters originate from weakly oxidizing to weakly reducing environment. Rich clays will contribute for gas sorption but hinder reservoir fracturing. Various kinds of shale pores within nanometer to micrometer scale are well developed. Longtan shales show good potential of shale gas but great production difficulty.