Geochemistry and origin of deep-seated cracked gas on the northern slope of the Dongying Sag, Shandong Province.

Deep-seated cracked gas exploration was achieved great breakthrough in Es of the Minfeng area on the northern slope of the Dongying Sag. Carbon isotopic and molecular compositions studies revealed the characteristics of wet gas and a normal trend of carbon isotopic composition. Empirical cutoff points of δC and δC and light hydrocarbon compositions distinguished the cracking gas as sapropelic gas. Variations in i/ nC, i/ nC and δC further confirmed that the gas was cracked from residual kerogen. Source characteristics indicated that the gas was derived from mature-highly mature source rocks of Es with kerogen type II being dominant mixed with some oil-cracking gas. Burial history modeling indicated that there were two hydrocarbon charging periods in Es reservoirs. The first period refers to the Guantao-Minghuazhen stage dominated by oil charging, while the second period refers to the Minghuazhen stage and has been dominated by cracked gas charging till now. [ABSTRACT FROM AUTHOR]