Secondary alteration of ancient Shuntuoguole oil reservoirs, Tarim Basin, NW China

Multiple tectonic movements and charge episodes have occurred in the Tazhong Uplift of the Tarim Basin, NW China, contributing to widespread solid bitumen, heavy oil, normal oil, condensate and gas in the Shuntuoguole High, especially in the Lower Silurian Kepingtage Formation. A detailed investigation of cores and fluid inclusions from different intervals, i.e. S(1)k(3-3), S(1)k(3-1), has been carried out using petrographic analysis, gas chromatograph-mass spectrometry and micro-Fourier transform infrared spectroscopy. Three hydrocarbon charge events in the Kepingtage Formation have been recognized associated with the late Caledonian, late Hercynian and Himalayan orogenies. In the late Caledonian, oils were charged into the S(1)k(3-3), S(1)k(3-1) and S(1)k(1) intervals, but evolved to solid bitumen due to biodegradation and water washing, as the Silurian strata have experienced both strong uplift and erosion during the early Hercynian Orogeny, and the influence of igneous intrusions during the Permian. Hydrocarbons associated with the Caledonian contain no regular steranes, diasteranes, hopanes or extended tricyclic terpanes due to in-reservoir thermal cracking. The second charge period occurred in the late Hercynian, which has experienced severe biodegradation and water washing. Hydrocarbons of this period are characterized by an obvious chromatographic hump of the unresolved complex mixture (UCM) coupled with petroleum inclusions that fluoresce light-blue and yellow, possibly indicating variable maturity. Whether or not high maturity oil charged during this period into the S(1)k(1) interval has commercial production value needs further investigation. The third change occurred during the Himalayan Orogeny. Highly mature hydrocarbons were emplaced into the S(1)k(3-3), S(1)k(3-1) and S(1)k(1) intervals with the S(1)k(1) interval as being the most significant. Biodegradation, water washing, and igneous intrusion had no impact on hydrocarbons charged during this period, resulting in higher potential for economic production.