Diagenetic evolution and its impact on reservoir quality of tight sandstones: A case study of the Triassic Chang 6 Member, Ordos Basin, northwest China. (July 2020)

• Record Type: Journal Article
• Authors: Yang, Shangfeng; Bao, Zhidong; Wang, Nan; Qu, Xuefeng; Lin, Yanbo; Shen, Jinjiang; Awan, Rizwan Sarwar
• Is Part Of: Marine and petroleum geology. Volume 117(2020)
• Publication Date: 2020-07
• Subjects: Tight sandstone -- Diagenesis -- Reservoir quality -- Triassic Chang 6 Member -- Ordos Basin; 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.2020.104360
• Languages: English
• ISSNs: 0264-8172

Abstract

Abstract: The Triassic Chang 6 Member sandstones are important oil reservoirs in the Ordos Basin of north-western China, and are actually considered to be typical tight sandstone reservoirs. Diagenetic alterations play a prominent role in considerably impacting the reservoir quality. In this study, an integrated approach of: thin sections, scanning electron microscopy (SEM), X-ray diffraction (XRD), cathodoluminescence (CL), fluid inclusions, laser Raman spectrographic analysis, and stable isotope analysis was applied to examine the diagenetic history recorded in the Chang 6 tight sandstones and investigate the influences of diagenesis on reservoir quality. The sandstones consist of fine-grained, moderated-to-well sorted arkoses (Q48.5 F40.9 R10.6 ) and lithic arkoses (Q54.3 F29.1 R16.6 ) with an average porosity and permeability of 9.10% and 0.39 mD, respectively. The sandstones have undergone complex diagenetic processes; dominant eogenetic features include: (ⅰ) mechanical compaction, (ⅱ) leaching of feldspar in meteoric water, (ⅲ) chlorite coats originating from the transformation of precursor clays and precipitating directly from interstitial water, (ⅳ) calcites filling intergranular pores as poikilotopic cements, and (ⅴ) quartz and albite overgrowths on surfaces of grains. Subsequent mesogenetic events include: (ⅰ) ankerite cements filling porosity and replacing framework grains, (ⅱ) extensive dissolution of feldspars and precipitation of kaolinites due to emplacement of organic acids, and (ⅲ) secondary quartz overgrowths in the late period of mesogenesis. Early calcite is characterized by 13 C (δ 13 CPDB ranging from −3.45‰ to −0.30‰, mean = −2.00‰) and 18 O (δ 18 OPDB varying from −16.70‰ to −5.80‰, mean = −10.50‰), while late ankerite is rich in 12 C (δ 13 CPDB ranging from −10.71‰ to 0.80‰, mean = −3.58‰) and 16 O (δ 18 OPDB varying from −24.30‰ to −15.80‰, mean = −19.93‰). Moreover, calcite hosts single liquid fluid inclusions, two-phase fluid inclusions are contained in ankerite cements (with mean homogenization temperature (Th) of 96.0 °C and salinity estimates from 11.47 to 18.96 eq. wt.% NaCl). These data indicate that the calcite originated from meteoric water and the ankerite was associated with organic matter and recrystallization of the early calcite cements. Furthermore, the metallic ions such as Fe 2+, Mg 2+, and Ca 2+ required for ankerite precipitation were produced through alterations of volcanic fragments, feldspars, and biotites. In comparison, compaction has more important role than cementation in the loss of original porosity although cementation exerts enormous influence on reducing porosity. Quantitatively, the compactional porosity loss reaches 54.9%, while the cementation porosity loss accounts for up to 42.8%. Carbonate cements and the other authigenic products (quartz and albite overgrowth, and kaolinite) mostly fill intergranular pores, thus reduce porosity. Chlorite coats have effects in inhibiting compaction and restraining the development of quartz overgrowth, thereby protect the intergranular porosity to some extent. Early secondary porosity was enhanced by leaching and dissolution of feldspar in the eogenetic period. However, these processes did not increase the net porosity in the mesogenetic stage. Highlights: Reconstruct diagenetic history based on dynamic analysis and test data. Interpret material sources of carbonate cement by isotope and petrologic data. Estimate leaching and dissolution in different stages for net porosity improvement. Discuss diagenetic events on tight sandstone reservoir quality quantitatively.