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Junggar Basin
The Junggar Basin is one of the most essential hydrocarbon‐bearing basins in north‐ western China, with an area of 13.6 × 104 km2. The Junggar Basin is located in the west of the Junggar–Tuha Plate and connects with the Western Junggar Orogenic Belt, as shown in Figure 1. Many oil and a few gas fields have been discovered through some decades of hydrocarbon exploration (Figure 1). Much progress has been made and commercial shale oil flow has been recently obtained from the Permian rock units. There‐ fore, the Junggar Basin has become a hot topic of petroleum exploration in China.
Figure 1. Location of Junggar Basin and study area in the Basin (Modified after reference). (a) geotectonic setting of the Junggar Basin; (b) tectonic unit of the Junggar Basin; (c) study area tec‐ tonic unit and petroleum, exploration well distribution. Reprinted with permission from Ref. 2020, Elsevier.
The basement of the Junggar Basin is of the Carboniferous age. The sedimentary fill of this basin comprises sedimentary units ranging in age from the Permian to the Quaternary, as shown in Figure 2. The Carboniferous stratigraphy of the Junggar Basin mainly deposits volcanic rocks and shallow marine clastic rocks. In the Early and Middle Permian, the Junggar Basin developed several depositional and subsidence centers, mainly fan‐deltaic and lacustrine sediments, which are the most critical source rocks. From Late Permian to Triassic, the Junggar Basin underwent a high subsidence phase. The Late Permian basins have a reduced distribution and a limited distribution of hydrocarbon source rocks. The Triassic basin is more extensive than the Upper Permian, and a basin‐covering layer system developed. During Jurassic times, a set of continental coal‐bearing deposited and associated shallow water sediments were deposited.
Figure 2. Lithological stratigraphy in the Junggar Basin (After reference). Reprinted with per‐ mission from Ref. 2020, Elsevier.
Source Rocks Geochemical Characteristics and Distribution As mentioned above, the natural gas generation potential of the Permian source rocks is the key to achieving breakthrough in natural gas exploration. The Permian source rock mainly consists of the Jiamuhe Formation, the Fengcheng Formation, and the Lower Wuerhe Formation. Since the distribution of hydrocarbon source rocks of the Jiamuhe Formation are relatively limited and the quality of source rocks of the Lower Wuerhe Formation are poor, only the geochemical characteristics of source rocks in the Fengcheng Formation are discussed. The geochemical features of source rocks include organic matter abundance, type, and maturity.
Organic Matter Abundance
Organic matter abundance is one of the main indexes to determine the hydrocarbon generation potential. At present, the commonly used organic matter abundance indexes mainly include total organic carbon content (TOC), chloroform asphalt “A”, and total hydrocarbon content (HC), as well as rock pyrolysis hydrocarbon generation parameters S1 (free hydrocarbon) and S2 (pyrolysis hydrocarbon), among which organic carbon content (TOC) and rock pyrolysis hydrocarbon generation parameters are commonly used.
The source rocks of the Permian Fengcheng Formation were widely drilled in the Mahu Sag. The TOC ranges from 0.1% to 4.0%. The main distribution of S1 + S2 ranges from 0.02 mg/g to 30 mg/g, suggesting that the source rocks are relatively fair, and good to excellent, according to the guidelines of reference.
Organic Matter Types
The types of kerogen have an obvious influence on the hydrocarbon generating ability. Type I kerogen has high original hydrogen content, low oxygen content, and the most substantial oil generating ability. The hydrogen content of type II kerogen is higher but slightly lower than that of type I kerogen. Type III kerogen has low original hydrogen content and high oxygen content. Its oil generating ability is poor, and it primarily generates natural gas.
The kerogen elemental diagram is the standard method to determine the type of organic matter. The H/C of kerogen in the Fengcheng Formation source rock is as high as 1.8. The main types of kerogen are type I and type II1, and the pro‐ portion of type II1 organic matter is relatively high, mainly humic organic matter. At the present phase of the hydrocarbon exploration no exploratory well has been drilled to the center of hydrocarbon producing sag. The source rock samples are mainly distributed in the slope area with shallow water bodies.
Organic Matter Maturity
Vitrinite reflectance (Ro) is the most commonly used index to determine the maturity of organic matter. Ro of hydrocarbon source rocks varies widely in the central depression of the Junggar Basin, up to 2.0% or more. The maturity of the source rocks from the Fengcheng Formation varies greatly and low to high maturity stage have been measured. It is generally believed that source rocks in the Fengcheng Formation had entered the phase of oil generation in the Mahu, Penyijingxi, and Shawan sags.
Distribution of Source Rocks
The source rocks revealed by drilling are mainly distributed around sag, and distribution of source rocks in sag is mainly predicted based on seismic due to lack of drilling data. The quality of seismic data has been poor for a long time and it isn’t easy to characterize the distribution of source rocks in the Junggar Basin. Several basin‐level 2D seismic grid lines were acquired in the Junggar Basin from 2018 to 2020. The imaging quality of deep seismic data was significantly improved through the combined processing of 2D survey lines and 3D seismic. The distribution characteristics of mudstone can be defined through the description of formation thickness. We consider the mudstone thickness to characterize the hydrocarbon source rock distribution because most of the mudstone samples of the Fengcheng Formation have TOC’s greater than 0.5%.
The distribution of the Permian source rocks in the western part of the basin is de‐ scribed. The source rocks of the Permian Fengcheng Formation are widely distributed in the three sags of the basin’s western region, with a thickness more than 100 m.
Figure 3. Source rock thickness isoline map of the Permian Fengcheng Formation in the Junggar Basin
In summary, the Permian source rocks in the Junggar Basin have high organic matter abundance and are mainly medium‐good hydrocarbon source rocks, with organic matter type II, and are developed from low to high maturity. They are primarily distributed in the Mahu, Penyijingxi, and Shawan sags in the western part of the basin.
Gas Generation Potential
Several decades’ petroleum exploration had proved the Fengcheng Formation hydrocarbon source rock potential, being the principal source of the billion‐ton big oil play in the central area of the Mahu Sag. Although early explorations have confirmed that as‐ sociated gas is common in the oil from the Fengcheng source rocks, there is still a lack of relevant research on whether the source rocks of the Fengcheng Formation can generate gas on a large scale.
Natural Gas Exploration Potential
The Permian Fengcheng Formation source rocks in the Junggar Basin have a high abundance of organic matter, suitable organic matter types, and moderate organic matter maturity evolution. Based on the study of geochemical characteristics, distribution, and gas generation potential, it could be concluded that the Fengcheng source rock can gener‐ ate medium to large‐scale natural gas reservoirs in the depths of more than 6500 m. The burial depth of the Fengcheng source rocks in the Mahu Sag is generally less than 6500 m, and several decades’ hydrocarbon explorations show that the resources are mainly oil and less natural gas. Despite the number of exploration wells drilled in the sag for shale oil exploration, no large‐scale natural gas reservoirs have been found. In the Penyijingxi and Shawan Sags, the burial depth of the Fengcheng Formation source rocks is at depth between 8500 m and 10,000 m, respectively. As the burial depth is far greater than 6500 m, it could be concluded that the Fengcheng source rocks in the Penyijingxi and Shawan Sags have great potential of natural gas generation. Recently, nat‐ ural gas reservoirs have been found around the Penyijingxi and Shawan sags, and several exploration wells have achieved the industrial gas flow standards in their production. In summary, the oil and natural gas exploration status is consistent with the research on the natural gas generation potential of Fengcheng Formation source rocks. Furthermore, in addition to the Fengcheng Formation source rocks, the burial depth of Jurassic source rocks in the southern margin of the Junggar Basin is over 10,000 m. And the Jurrassic coal‐bearing source rocks are widely distributed, with high organic matter abundance, mainly Type‐II and Type‐III organic matters, and high natural gas generation potential. In summary, both the Permian source rocks in the Penyijingxi and Shawan sags and the Jurrassic source rocks in the southern margin of the Junggar Basin have a relatively higher potential of natural gas generation.
Natural Gas Exploration Potential
Historically, Junggar Basin is rich in oil and poor in natural gas, and usually consid‐ ering as a typical “oil basin”. The ratio of proved natural gas reservoirs to oil reservoirs being only 0.06:1, and the natural gas exploration has seen no breakthrough until 2019. Since 2020, several exploration wells production have achieved the industrial gas flow standards, the natural gas exploration potential should be paid attention. Around the Penyijingxi and Shawan Sags, several exploration wells’ (SX 16, SX 18, etc.) production have achieved the industrial gas flow standards. In the southern margin of the Junggar Basin, a significant breakthrough in natural gas exploration has been achieved by the exploration wells GT 1 and HT 1, which confirms the sizeable natural gas exploration potential in the southern margin of the basin. In summary, the oil and natural gas exploration status is consistent with the research on the natural gas generation potential of the Fengcheng Formation source rocks, and it is believed that the Junggar Basin may have huge natural gas exploration potential.
Data source: Gas Generation Potential of Permian Oil‐Prone Source Rocks and Natural Gas Exploration Potential in the Junggar Basin, NW China. Abilimit Yiming, Xiujian Ding, Liangrong Qian, Hailei Liu, Maoguo Hou and Zhongfa Jiang. 2022
Следующий Бассейн: Melawi