Search

With the continuous increase in exploration efforts in new zones and new strata, significant breakthroughs have been made in the natural gas exploration of the O1m56 to O1m4 formations in the Ordos Basin. Thus, the origin and exploration potential of subsalt natural gas have attracted much attention and urgently need to be addressed. On the basis of certain geochemical characteristics, genetic types, and sources of natural gas, a comprehensive study on the sedimentary environment, organic geochemical characteristics, and spatial distribution scale of source rocks are conducted in this paper by using geological and geochemical methods. The study shows that: (1) The Ordovician subsalt natural gas is mainly “pyrolysis dry gas,” among which the δ13C1 of Ordovician subsalt low sulfur (sulfur-free) natural gas is lighter, with an average value of −39.6‰; the δ13C2 ranges more largely from −35.6‰ to −25.8‰. In contrast, both δ13C1 and δ13C2 values are heavier in high-sulfur natural gas, revealing that different Thermochemical Sulfate Reduction (TSR) reaction stages have different degrees of influence on natural gas components and carbon isotope composition. (2) Subsalt natural gas is classified as “oil-type gas,” which is self-generated and self-accumulated, whose source rocks are mainly Ordovician subsalt marine deposits. (3) Three types of marine source rocks are developed in Ordovician subsalt, including black argillaceous rock, dark argillaceous dolomite (dolomitic mudstone), and dark micrite (bioclastic) limestone. In addition to micrite limestone, these rocks were mainly formed in a confined lagoon sedimentary environment with high salinity and anoxia. Sedimentary water was significantly stratified and the environment was highly reduced. The organic matter content of the source rocks is relatively high, with an average TOC value of 0.45%. The hydrocarbon-generating parent materials are mainly composed of bacteria and algae, and the organic matter evolution reaches high-over maturity stage. The total gas generation amount of the marine source rocks in Ordovician subsalt is approximately 43.8 × 1012 m3, which can provide hydrocarbons and accumulate for the subsalt favorable reservoir facies located far from Upper Paleozoic gas sources.

Water, in ultra-deep layers of the earth and in layers receiving abnormally high heat, can exist in a supercritical state. Supercritical water (SCW) can participate in the transformations of organic compounds not only as a solvent but also as a reactant, influencing petroleum formation and the evolution of sedimentary organic matter. Here, we carried out hydrous pyrolysis experiments in both SCW and water vapor (WV) using two organic-rich marine rocks under closed conditions, to quantitatively evaluate the generation potential of hydrocarbons in a supercritical state and to clarify the effect of water phase on hydrous pyrolysis experiments. The results showed that SCW promoted gaseous and liquid hydrocarbon generation and facilitated the cracking of aliphatic hydrocarbons. For gaseous hydrocarbons, the action of SCW became stronger as the temperature increased. For liquid hydrocarbons, the peak yields of bitumen were enhanced by the SCW, and the temperature corresponding to peak yield in SCW was lower than that in WV. These results were attributed to the supply of hydrogen and oxygen from SCW for the petroleum formation and cracking. The δ13C and δD values of gases were also influenced by SCW. However, these values obtained in SCW did not always become positive as their yields increased. Generally, methane (CH4) was enriched in 12C and 1H. The influence of SCW on the isotopic fractionation of ethane (C2H6) and propane (C3H8) was more complex. The water phase is an important factor affecting the experimental results of hydrous pyrolysis. From these findings, it can be concluded that SCW allowed for increased conversion of sedimentary organic matter to gaseous and liquid hydrocarbons in ultra-deep layers and layers affected by volcanic-hydrothermal activity.

In recent years, the northwestern part of the Sichuan Basin has become a popular location for natural gas exploration. However, up to this point, the geochemical characteristics and distribution of marine source rocks in this area have not been thoroughly investigated. In the northwestern Sichuan Basin, there are eight sets of marine source rock, including the Lower Sinian Doushantuo Formation, the third Member of Upper Sinian Dengying Formation, the Lower Cambrian Qiongzhusi Formation, the Lower Silurian Longmaxi Formation, the Middle Devonian, the Middle Permian Maokou Formation, the Upper Permian Longtan Formation/Wujiaping Formation, and the Upper Permian Dalong Formation, from which eight outcrop profiles were selected for systematic organic geochemical analysis. The Qiongzhusi Formation has the thickest high-quality source rocks, up to 80 m thick, followed by the Upper Permian Longtan/Wujiaping and Dalong formations, which have up to 20–30 m thick high-quality source rocks, whereas the thickness in other layers is relatively thin. By the abundance of organic matter, the Doushantuo, Qiongzhusi, Longmaxi, Longtan/Wujiaping, and Dalong formations developed high-quality source rocks with total organic carbon content (TOC) greater than 3%, while the third Member of Upper Sinian Dengying Formation, Middle Devonian and Maokou formations had low TOC. The kerogen of source rocks gradually enriched 13C from the type Ⅰ organic matter of the Lower Cambrian Qiongzhusi Formation to the Upper Permian Dalong Formation, and the δ13C values of Doushantuo Formation, the third Member of Dengying Formation, and Qiongzhusi Formation are less than −30.0‰. The δ13C values of Longtan Formation or Wujiaping Formation and Dalong Formation with type Ⅱ2 organic matter ranges from −28.0‰ and −26.0‰. Others lie somewhere in between type I and type II2. Except for Permian source rocks in Guangyuan and Chaotian areas, which are in the mature stage, source rocks in the northwestern Sichuan Basin are in the over-mature stage and largely in the thermal stage of dry gas. High abundance source rocks have provided rich marine natural gas resources. As a result, future gas exploration in the northwestern Sichuan Basin will focus on the gas-bearing system associated with the Qiongzhusi Formation and Upper Permian source rocks.

To clarify the organic matter (OM) enrichment of the Lishui Sag, the factors influencing the variable abundance of OM in the Lingfeng Formation are studied using organic geochemical data. The source rocks of the Lingfeng Formation have medium–high total organic carbon (TOC) values (0.53–3.56%). The main type of kerogen is II2-III. Compared to the shallow marine subfacies source rocks, the TOC of the delta front subfacies source rocks is higher. The distribution of biomarkers shows that the redox environment of the delta front subfacies source rock is the sub-oxidizing and oxic environment, and the source rock is mainly supplied by terrigenous higher plants; the redox environment of shallow marine subfacies source rocks is a sub-reducing and suboxic environment, and the OM mainly comes from algae. The link between OM input and OM abundance demonstrates that terrigenous OM (TOM) input has a considerable influence on OM abundance. However, there is no obvious relationship between preservation and OM abundance, which suggests that preservation is not the determining element in OM enrichment. The strong sediment flux decreases the amount of time that OM is exposed to oxygen. As a result, delta front subfacies with large TOM input have a huge number of excellent source rocks. This paper proposes a “delta front-OM input model” for excellent source rocks.