Abstract
Plenty of marine crude oil generated by the Middle–Upper Ordovician source rocks has been discovered in the Tabei Uplift, Tarim Basin. Previous studies on the extensively distributed low organic matter source rock total organic carbon ≤ 0.5%) imply that this contributes to oil accumulation. Thus, the identification of present-day low-total organic carbon source rocks that experience hydrocarbon expulsion and evaluation of their contribution is of considerable significance to analyze marine oil accumulation and to assess the resources in the Tarim Basin. Using the mass balance approach, we identify and verify present-day low-total organic carbon carbonate source rocks and present a quantitative model using well-logging data to predict the present-day low-total organic carbon carbonate source rocks in order to evaluate their contribution in the Tarim Basin. Hydrocarbon expulsion has occurred in the Yijianfang formation, which has present-day low-total organic carbon, between 6452 and 6487 m in the Yangwu 2 well. In the horizons depicting carbonates with hydrocarbon expulsion, the porosity is relatively high and the daily oil production is 0.19 m3 according to the perforation tests. The extracts have similar biomarker signatures to that of the crude oils in the Tabei Uplift. In this study, a quick and cost-effective quantitative model is developed using double lateral resistivity and uranium/thorium well-logging data. By applying the prediction model, the contribution of present-day low-total organic carbon carbonate source rocks in the Middle–Upper Ordovician formation of the Tarim Basin is primarily evaluated to be 363.38 × 109 bbl oil equivalent.
Highlights
Using the improvements in the hydrocarbon exploration theory and technology, the exploration of the Tarim Basin in China has progressed
Recent data from 246 wells suggest that the Middle–Upper Ordovician carbonates are commonly observed to be too low in organic matter to contribute significantly to hydrocarbon accumulation in the carbonate platform successions of the basin (Figure 1)
From the third round of the national resource assessment based on the contribution of high organic matter Cambrian–Lower Ordovician source rocks, the predicted in-place resources of oil, gas, and oil equivalents in both the Tazhong and Tabei areas are observed to be lower than the discovered reserves (Yang, 2012, personal communication), implying a contribution by the present-day low-TOC carbonates
Summary
Using the improvements in the hydrocarbon exploration theory and technology, the exploration of the Tarim Basin in China has progressed. Recent data from 246 wells (some located on the slope close to the Manjiaer Depression, e.g. SINOPEC’s well SB1 and PetroChina’s well GL2) suggest that the Middle–Upper Ordovician carbonates are commonly observed to be too low in organic matter (present-day TOC 0.5%) to contribute significantly to hydrocarbon accumulation in the carbonate platform successions of the basin (Figure 1). From the third round of the national resource assessment based on the contribution of high organic matter Cambrian–Lower Ordovician source rocks, the predicted in-place resources of oil, gas, and oil equivalents in both the Tazhong and Tabei areas (the two richest hydrocarbon areas in the platform of Tarim Basin) are observed to be lower than the discovered reserves (Yang, 2012, personal communication), implying a contribution by the present-day low-TOC carbonates. With one cutting sample in each meter, 86 samples were taken
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