Abstract:

The Ordovician in the Tarim Basin is an important period of source rock and reservoir formations in the basin. The establishment of an isochronous stratigraphic framework for the Ordovician Yingshan, Yijianfang and Qrebake Formations in the Shuntuoguole area is the basis for geological research and discovery of oil and gas resources. In this contribution, based on the characteristics of petroelectricity, we established the standard of carbon isotope correlation, perfected the conodont zones from Ordovician platform to slope facies, established the division correlation framework of the Ordovician Yingshan-Qrebake Formation and revealed its features and distribution patterns. In perfecting the sequence of conodonts zones, we found that the carbon isotope obviously changed at the boundaries of the three formations, and the stratigraphic sequence was well developed. The sedimentary period of the Yingshan Formation was the peak period of platform development in the Tarim Basin. The sedimentary environment was stable, the water depth basically unchanged, and the carbonate sedimentary rate was relatively high and remained nearly constant. The Yingshan Formation was about 700 m thick in general and its distribution relatively stable. It can be divided into four members, each member was 130-150 m thick. The thickness of the Yijianfang Formation varied from 100-250 m. The Shunnan section was the thickest, with thickness generally over 200 m and possibly reaching 250 m; the Gulong and Shunbei sections came in second, they were generally 130-150 m thick; and the Yuejin section was the thinnest, only 100 m thick. During the sedimentary period of the Yijianfang Formation, the sedimentary environment of the Shuntuoguole area was a carbonate gentle slope environment, and the carbonate sedimentary rate was relatively low. The Yijianfang Formation can be divided into two members: the ~60-70 m thick upper member and the ~70-80 m thick lower member. During the sedimentary period of the Qrebake Formation, the study area was in a deep-water slope environment, with low sedimentary rate, stable sedimentary environment and uniform thinness, generally only about 30-40 m thick, over the whole area.

Key words: carbon isotope, conodont zones, Ordovician, Yingshan Formation, Yijianfang Formation, Shuntuoguole area, Tarim Basin