| Hydrocarbon exploration and geological study in deep-buried strata are an important frontier field currently. Such strata in the Mahu sag of the Junggar Basin (> 5000 m), NW China have been predicted to have good resource prospects and are an important exploraiton target regionally or even domestically. However, the exploration and geological study are scarce. Therefore, in order to provide scientific foundation for the next-step exploration and new reference information for other studies with similar geological setting, we address the hydrocarbon source rock and accumulation model, based on comprehensive and comparative geological and geochemical studies. Three new lines of advances and understandings were obtained.(1) Hydrocarbon source rocks were reevaluated. Through a systematic and comparative analysis of geology and geochemistry of hydrocarbon source rocks and oil and gas from forward and reversal perspectives, we propose that the study area may contain three sets of potential hydrocarbon source rocks in addition to the high-quality lower Permian Fengcheng Formation source rocks as commonly known. The three new potential source rocks include the Carboniferous, lower Permian Jiamuhe Formation, middle Permian Lower Wuerhe Formation. All the four sets of hydrocarbon source rocks have a high abundance of organic matter reaching medium to good quality, various kerogen types and moderate to high maturity. As a consequence, they can generate both oil and gas. Therefore, the hydrocarbon resource potential of the deep-buried strata in the study area is better than previously thought, and there is good hydrocarbon resource conditions.(2) Oil and gas nature and type were predicted. This was made through a study of buried-thermal evolution of hydrocarbon source rocks in the study area and oil and gas geochemical analysis. Results indicate that the hydrocarbon is highly mature in nature and light in general and includes various types, e.g., highly mature and light oil, kerogen cracking gas and oil cracking gas. Therefore, in the deep-buried strata, the oil is light, highly-mature and has good preservation condition, while it is likely for highly-over mature gas accumulation, which is beneficial to the large-scale hydrocarbon accumulation and enrichment. We further discuss the controlling factors of the oil and gas nature and type. It is suggested that the static reservoir property and pressure conditions and the dynamic migration conditions are important.(3) Hydrocarbon accumulation model was established. Through analyses of hydrocarbon sources and charging stages/time, combined with geological setting, it is showed that the deep-buried oil and gas in the study area are mainly derived from the lower Permian Fengcheng Formation and the reservoir experienced two stages of hydrocarbon charging and one stage of hydrocarbon dysmigration/remigration. Specifically, the first stage of hydrocarbon accumulation took place at the end of the Triassic, when mature oil from Fengcheng Formation the charged reservoir, and the reservoirs were protected. During the Middle-Late Jurassic, these mature oils were subject to degradation and adjustment under the influence of Yanshannian movement, and preserved in the reservoirs in the form of residual hydrocarbons. The second stage of hydrocarbon accumulation occurred during the Early Cretaceous, the Fengcheng-sourced oils and gas charged the reservoirs. Over pressure was generally developed, beneficial to protect the reservoir quality and hydrocarbon accumulation.Integrated the results and understandings above, we can conclude that in the studied Mahu sag, the deep-buried strata have good resource conditions, the hydrocarbon nature is light, and the accumulation conditions are excellent. Therefore, the deep-buried strata are worthy of attention in the next-step exploration. |
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