Apparent phenomenon and origin of "oil-cracking gas" in Kelasu structural belt of Kuqa Depression

ObjectiveThe Kelasu structural belt of the Kuqa Depression in the Tarim Basin hosts abundant deep to ultra-deep natural gas resources, and the reservoirs generally exhibit a high degree of thermal maturity. Natural gas from the Kela-Keshen area exhibits a characteristic "oil-cracking gas" signature on the conventional genetic diagram of ln(C2/C3) versus ln(C1/C2). However, this interpretation is inconsistent with the geological reality, as the area lacks effective Type Ⅰ-Ⅱ kerogen and oil sources, resulting in a discrepancy between the inferred gas origin and the actual geological conditions. To elucidate the apparent “oil-cracking gas” phenomenon and its genetic mechanism in this area, an in-depth investigation of the geochemical characteristics is required. MethodsTwo representative blocks along the East-West trend of the Kelasu structural belt (Bozi and Kela-Keshen) were selected as study areas. Gold-tube thermal simulation experiments, natural gas compositional analyses, and carbon isotope analysis of natural gas were conducted to systematically investigate the thermal evolution characteristics of natural gas derived from different source rocks. On the basis of the ln(C2/C3) versus ln(C1/C2) crossplot, a quantitative analytical model for mixed-source interpretation of natural gas in the Kelasu area of the Kuqa Depression was established. ResultsButane and pentane isomerization parameters (iC4/nC4 >1.0 and iC5/nC5 > 1.0) indicated that natural gas in the Kelasu area was kerogen-cracking gas. Mixed-source proportion calculations revealed that natural gas in the Kela-Keshen area was primarily derived from mature coal-bearing source rocks, with contribution ratios commonly exceeding 80%. In contrast, natural gas in the Bozi area was mainly sourced from lacustrine mudstone, with coal-derived gas contributing only approximately 30%. ConclusionsNatural gas in the Kelasu area is not oil-cracking gas, but rather represents cracking products from different kerogen types at high to over-mature stages. The apparent "oil-cracking gas" characteristics displayed on classical identification plots result from the high degree of thermal evolution. The results reveal the complexity of deep natural gas genesis under high evolution conditions, and establish the quantitative analysis method of mixed source gas, which provides a theoretical basis for deep natural gas exploration in Kuqa Depression and similar high evolution basins.