Pyrite Characteristics in Lacustrine Shale and Implications for Organic Matter Enrichment and Shale Oil: A Case Study from the Triassic Yanchang Formation in the Ordos Basin, NW China

Pyrite is widely distributed in lacustrine shales and has become a research focus in unconventional oil and gas exploration. Pyrite morphology is useful for identifying different types of organic matter and assessing shale oil enrichment in organic-rich shale. Abundant pyrite is developed in the source rocks from the Chang 7 Member of the Yanchang Formation in the Ordos Basin, NW China. However, the relationship between different pyrite types and the differential enrichment of shale oil still needs to be clarified. The organic geochemistry, petrology, and isotopic composition of the Chang 7 Member samples were analyzed. The significance of pyrite types and sulfur isotopic compositions as indicators of depositional environments and shale oil enrichment was emphasized. The Chang 7 shales contain three pyrite morphologies, framboidal pyrite (type A), spherulitic pyrite (type B), and euhedral and anhedral pyrite (type C), and their aggregates. The sulfur isotopic compositions of pyrite (delta S-34(py)) in Chang 7 shales with different pyrite types exhibited regular patterns. The delta S-34 values of types A, B, and C pyrites were sequentially positive overall (average values are -2.739, 2.201, and 7.487 parts per thousand in sequence), indicating that type A pyrite was formed during the syn-sedimentary to early diagenetic stage and types B and C pyrites were formed during the early to middle diagenetic stage. Types A, B, and C pyrites showed sequentially increasing kerogen type index values and kerogen carbon isotope values (mean values of -31.59, -28.70, and -26.45 parts per thousand, successively), indicating that the horizons where types A, B, and C pyrites developed correspond to types I, II, and III organic matter, respectively. Strong correlations between the pyrite content and oil components reveal that pyrite indicates shale oil enrichment. Moreover, variations in pyrite type significantly influenced the enrichment behavior of shale oil. Types A and B pyrites contributing to reservoir space showed shale oil enrichment. They promoted saturated hydrocarbon enrichment at >15% pyrite content, whereas type C pyrite did not indicate shale oil enrichment. These findings provide new insights into the differential enrichment of organic matter and shale oil and valuable guidance for the large-scale exploration and development of shale oil resources.