Logging inversion geochemical parameters of hydrocarbon source rocks:A case study of Jurassic lower Xinhe Formation in Xiaohu sub-sag,Yabulai Basin

The abnormal high pressure is common in the source rock of mudstone because of differential compaction and hydrocarbon generation. In order to avoid the significant error caused by the sonic decompaction correction in hydrocarbon source rock evaluation,it was found that the response of natural gamma logging is obvious for organic and is not affected by formation compaction based on the logging rock physical response mechanism and the curve response characteristics. Therefore the organic carbon content of hydrocarbon source rocks can be better obtained than that using traditional methods by the congruent natural gamma and deep resistivity curve. Meanwhile the inversion model of organic carbon content was established by multiple nonlinear regressions. According to the organic carbon content distribution and the formation compaction degree,the inversion model error on organic carbon content was analyzed taking the determined TOC data as the model check normal. Results show that the accuracy of inversion results on organic carbon content is high,especially it is significantly higher than that using traditional methods on the rich quality hydrocarbon source rock (TOC> 1%),and has not significant difference on the poor quality hydrocarbon source rock(TOC< 1%). On the other hand,the inversion result is more reliable than that of traditional methods in the medium and strong abnormal pressure hydrocarbon source rock (surplus pressure> 15 MPa),and there are no significant discrepancies on the normal and slight abnormal pressure (surplus pressure< 15 MPa). Then,according to the congruent distance between natural gamma ray curve and deep resistivity curve,the log inversion models of hydrocarbon generation potential and hydrogen index were established respectively,and at the same time,the base was set up. The maximum peak temperature profile of pyrolysis can be obtained from the mathematical statistical relationship between depth and maximum peak temperature of pyrolysis. Then the kerogen type and organic matter abundance can be quantitatively evaluated by using the organic carbon content,hydrocarbon generation potential,maximum peak temperature of pyrolysis and hydrogen index retrieved from logging data,and the vertical continuous heterogeneity characteristics of geochemical parameters can be defined. Finally,according to the quality evaluation criteria of hydrocarbon source rocks,the developed thickness of high quality source rocks at stratigraphic sections in single well can be interpreted.