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Prediction of Source Rock Character from Chemical and Isotopic Composition of Ordovician Reservoired Oils in the Tarim Basin, NW China

Maowen Li
Sinopec Petroleum Exploration & Production Research Institute, Beijing, China

In sparsely explored or frontier basins, petroleum source rocks are usually unknown, or only a limited number of crude oils or seep samples may be available for study. Various geochemical methods can be applied to such oils in attempts to provide information on their source rocks from oil composition, such as the organic matter type, depositional environment, and mineralogy of the source rock. This information is useful to geologists who plan further exploration within a basin. The source of Ordovician-reservoired oil in the Tarim Basin, NW China remains contentious due to the scarcity of deep wells that penetrate the likely source rock intervals and the relatively high thermal maturity of the potential source rocks revealed from outcrop samples. Nevertheless, considerable progress has been made in the last decade in the identification of oil source rocks in the Tabei and Tazhong Uplift regions. Earlier studies proposed that the Cambrian–Lower Ordovician strata might be the source of the oils in the Tabei Uplift, because of the limited penetration of Middle-Upper Ordovician source rocks and their low organic abundance. However, more recent studies emphasized the contribution from the Middle-Upper Ordovician source rock, based on the distribution of dinosteranes, triaromatic dinosteroids, 24-isopropylcholestane and 24-norcholestane. This study attempts to integrate various geochemical methods of crude oil characterization in order to delineate the lithofacies information on their source rocks. Geochemical analysis indicates that crude oils in the Ordovician reservoirs in the Tabei Uplift were derived from marine carbonate source rocks deposited under anoxic conditions, displaying low pristine/phytane, C21/C23 tricyclic terpane and C28/C29 sterane ratios, high C29/C30 Hopane and C35/C34 Hopane ratios, a “V” shape in the distribution of C27-C28-C29 steranes and light carbon isotope ratios. These oils correlate well with the Middle-Upper Ordovician source rock, but some source-related biomarker parameters of the oils located to the south of the Tahe oilfield imply a more siliceous clastic source organofacies with high thermal maturity. The majority of oils in the Tazhong Uplift share typical biomarker characteristics from a marine carbonate source rock and related crude oil features. Only a few oils in the Tazhong Uplift display typical biomarker features previously attributed to a Cambrian–Lower Ordovician source. However, stable carbon isotope values of individual n-alkanes in most of the crude oils in the Tazhong Uplift appear to indicate a mixture of two end member oils. As it has been demonstrated that the Lower Palaeozoic organic-rich source rocks in the Tarim Basin were deposited in several depositional settings, it is critical to ascertain whether the end members represent two separate source lithofacies of the same geological age, or different facies of different ages.

AAPG Search and Discovery Article #90175©2013 AAPG Hedberg Conference, Beijing, China, April 21-24, 2013