Organic carbon, bitumen content and maturity of Lower Cambrian source rocks in the northern margin of Tarim Basin

Abstract The lower Paleozoic area in Tarim Basin is the key field of deep oil and gas exploration, and the lower Cambrian source rock is the focus of current research. However, due to the deep burial depth, there are few samples of Lower Cambrian source rocks revealed by drilling. The lower Cambrian source rocks are widely exposed in the northwest of the basin, which provides a good opportunity for the study of source rocks. In this paper, the organic geochemistry of three sections and a shallow drill sample from Aksu Area in the northwest and Kuruktag area in the northeast of the basin has been systematically analyzed. The total organic carbon (TOC), bitumen content and maturity of source rocks are studied. The organic carbon content of the lower Cambrian source rocks in the Keping area of the northwest basin ranges 1.49%-14.07%, and the content of bitumen ranges 7.73-24.53 mg / g • TOC. However, the organic carbon content of the lower Cambrian outcrop samples in the Sugetebulake section ranges from 0.93% to 7.90%, and the soluble organic matter content ranges from 0.73 to 5.89 mg / g • TOC. The TOC of Lower Cambrian source rocks in Yardang mountain area of northeast basin is low, ranging from 0.75% to 1.42%, and the content of bitumen is between 2.97-3.94 mg / g • TOC. In terms of organic matter abundance, the Cambrian source rocks developed in gentle slope facies are better than those in deep-water shelf facies. The measured reflectance (Ro%) of Shairik shallow drilling samples is between 1.0% - 1.7%, with an average of 1.45%. The Ro% of Sugatebulak samples is between 1.4%-1.75%, with an average of 1.55%, indicating that the overall maturity of the source rocks in the northwest is in the condensate wet gas stage. The measured reflectance of the Cambrian source rocks in the Yardangshan and Kuruktag sections in the northeast of the basin is between 1.5-2.5% RO, with an average of 1.75% ro. The overall maturity of the source rocks in the northeast of the basin is relatively higher, suggesting a wet gas stage.