鄂尔多斯盆地三叠系延长组黑炭的发现、地质意义及其对烃源岩评价的影响

Discovery and geological significance of black carbon in Triassic Yanchang Formation in Ordos Basin and its influences on source rock evaluation,NW China

通过对鄂尔多斯盆地三叠系延长组开展研究,首次在中三叠统长7段泥页岩发现黑炭,提出中三叠世东特提斯地区氧气含量超过15%且植物已经从二叠纪末大灭绝后复苏的观点。研究表明:①长7段黑炭的分布具有较强的区域差异性,盆地东南部泥页岩的黑炭含量最高、可超过6%,占总有机碳含量的比例最高为20%,而西北和东北部则低于10%。②传统上以TOC为关键参数的烃源岩评价技术和页岩油气选区方法面临考验:TOC值达标的泥页岩不一定是有效的烃源岩和非常规页岩油气有利目标,而低TOC也可能成为有效、甚至优质烃源岩。③将泥页岩总有机碳含量进一步精细划分为黑炭(w_(b))、活性碳(w_(a))、残留碳(w_(r))和成熟页岩油中的碳(w_(o))等质量分数,对于烃源岩评价推荐采用总有机碳含量减去黑炭含量作为指标,对于中低成熟度页岩油原位开采选区宜重点采用活性碳含量指标,对于中高成熟度页岩油的有利区选择则使用成熟页岩油中的碳。研究结果可以定量评价泥页岩有机质组成、烃源岩生烃潜力、烃源岩演化阶段以及页岩油生成、排出与滞留,并对烃源岩发育期的气候环境重建以及页岩油气甜点优选具有重要意义。

Through investigating the Triassic Yanchang Formation in the Ordos Basin,black carbon has been found for the first time in the seventh member of the Middle Triassic Yanchang Formation(Chang 7 Member).This study suggests that the oxygen content in the East Tethys during the Middle Triassic was beyond 15%and that plants had recovered from the Late Permian mass extinction.The results show that the distribution of black carbon in the Chang 7 Member is heterogeneous in the basin.In the southeastern part,the black carbon content is the highest(possibly higher than 6%)in shale,with the proportion in total organic carbon content(TOC)up to 20%,which is lower than 10%in the northwestern and northeastern parts.The traditional practice needs to be noticed when TOC is set as a critical index in source rock evaluation and shale oil and gas sweet spot screening.Shale with high TOC may not be effective source rock and good target for unconventional oil and gas exploitation,while shale with low TOC may become effective or high-quality source rock.The TOC in shale can be divided into mass fractions of black carbon(w_(b)),active carbon(w_(a)),residual carbon(w_(r)),and carbon from mature shale oil(w_(o)).TOC-wb is recommended for evaluation of source rock,wa for screening the in-situ recovery area of low to medium maturity shale oil,and wo for appraisal of the favorable exploration area of medium to high mature shale oil.These results allow for the quantitative evaluation of organic matter composition of shale,hydrocarbon generation potential,maturation stage,and generation,expulsion and retention of shale oil,and also guide the reconstruction of climate in the source rock development period and the shale oil and gas sweet spot screening.