156 / 2019-06-02 15:23:30

Petrological Evidence of Shallow-Water Onlap Deposition of Organic-Rich Fine-Grained Sediments — A Case Study of Cambrian Yuertus Formation in Tarim Basin

Key words: Fine-grained sedimentary rocks;Hydrocarbon source rocks;Shallow water onlap;The exposed surface; karst;Sedimentary model;Yuertus formation;Tarim basin

ABSTRACT Deep and shallow water disputes have existed for a long time in the formation environment of organic shale. Taking the outcrop profile of Cambrian Yuertus Formation in Aksu-Keping-Wushi area in northwestern Tarim Basin as an example, the macro and micro work of the outcrop profile shows that: (1) In the lower part of the Yuertus Formation, the organic-rich fine-grained sedimentary rocks or thin-rhythmic interbeds with siliceous rocks change upward into terrigenous clastic mixed granular beaches and reverse-grained carbonate rocks. (2) The thin limestone interbedded with dark shale rhythmically has inverse grain sequence. (3) Thin-bedded siliceous rocks have metasomatic residual grain structure and cementation fabric. (4) There are iron crust layers at the top of the shallowing sequence of the mixed granular beach, under which karst trenches, karst fissures, cystic karst caves, karst (plastic) breccia and terrigenous clastic fillings can be seen. (5) On outcrops and seismic profiles, fine-grained organic-rich sedimentary rocks above unconformities or exposed surfaces have typical overlap characteristics. Comprehensive analysis shows that the organic-rich fine-grained sediments of the Cambrian Yuertus Formation were formed in the anoxic-suboxidized restricted Gulf lagoon environment, and their formation may be controlled by high paleoproductivity and poor oxygen exchange, thus establishing a shallow overlap sedimentary model. The results will help to enrich and improve the sedimentary theory of fine-grained sedimentary rocks rich in organic matter. In addition, because the deposition and filling process indicates that the relatively large-scale organic rich shale distribution is related to geomorphic depressions, the microgeomorphic recovery in the sedimentary stage may be an effective means for shale gas deposition and reservoir prediction, so it also has important unconventional reservoir geological significance.