MULTIDISCIPLINARY IDENTIFICATION OF STRATIGRAPHIC REPETITION IN A SINGLE-WELL: A CASE STUDY OF TRIASSIC SEQUENCE OF WELL SHANKE-1, TURPAN-HAMI BASIN, NW-CHINA

Single-well stratigraphic division is the key to regional investigation of petroleum geology. Most of the previous single-well stratigraphic divisions were only based on palaeontology and lithology, which often resulted in uncertainties and multi-interpretations.

The Well Shanke-1, a scientific drilling project, is the first well which penetrated Triassic in Taibei Deprssion of Turpan-Hami Basin, Northwestern China. There is much controversy over stratigraphic division of the interval from 4555 to 5374 m in the well. Some geologists think that the 4555-5083 m and 5083-5374 m intervals are Triassic and Upper Permian respectively, and others consider the whole 4555-5374m interval as a successive Triassic sequence. This study concludes that the 4555-5083 m and 5083-5374 m are two repeated Triassic intervals, with a detachment fault at 5083m. This conclusion is supported by the following evidences: (1) The same Late-Triassic sporopollen assemblage, i.e. Aratrisporites ¨C Punctatisporites ¨C Caytonipollenites, occurs both in 4555-4823 m and 5083-5242 m intervals; (2) The lithological association of 4555-5083 m interval is similar to that of 5083-5374 m interval; (3) A detachment fault is found at 5083 m on the seismic sections across Well Shanke-1; (4) Dipmeter logging indicates abrupt changes of stratigraphic dip, dip angle, and borehole principal stress in 5050-5100m interval; (5) Similar increasing and decreasing patterns of inorganic element contents including Ni, Ga, Co, Fe, Al, Vand Rb, are found in 4555-5083 m and 5083-5374 m intervals; (6) Vitrinite reflectances (Ro) at 5086 m and 5093 m are lower than that in the upper intervals.

Multidisciplinary analysis has identified the stratigraphic repetition of the 4555-5374 m in Well Shanke-1. Integration of different methods and data has broken the old model of stratigraphic division using a single method, which effectively reduced the uncertainties, and hence provided us with a reliable division. We believe that such integrated approaches used in this study are highly applicable to the single-well stratigraphic division in other regions.