Symbiotic Combination and Accumulation of Coal Measure Gas in the Daning–Jixian Block, Eastern Margin of Ordos Basin, China

Abstract

Coal measure gas resources, including coalbed methane (CBM), shale gas, and tight gas are abundant in the Daning–Jixian Block. The complexity of the source–reservoir–cap relationship in the coal measure strata leads to unclear symbiotic characteristics and gas accumulation, which in turn, restrict the exploration and exploitation of the coal measure gas. In this study, the enrichment and accumulation of coal measure gas are discussed and summarized in detail. The results show that there are eight lithofacies and six reservoir combinations in the superposed strata of the coal measures in the study area. Controlled by the tidal flat-lagoon facies, the “sand-mud-coal” type mainly distributes in P1s2 and P1t, showing a good gas indication. Based on the variation of the total hydrocarbon content, key strata, and pressure coefficient of the coal measure gas reservoir, four superposed gas-bearing systems are identified in the vertical direction. According to the relationship between the gas-bearing system and gas reservoir, the enrichment of coal measure gas in the study area can be divided into three modes, including an intra-source enrichment mode, a near-source migration enrichment mode, and a far-source migration enrichment mode. The symbiotic accumulation of a coal measure gas model is further proposed, that is, an “Adjacent to co-source reservoir” type superimposed coalbed methane and shale gas reservoir model, a “Three gas symbiosis” superimposed reservoir model in the local gas-bearing system, and a “Co-source far reservoir” tight sandstone gas reservoir model. Clarifying the symbiotic relationship of coal measure gas reservoirs is beneficial to the exploration and further production of unconventional gas in the study area.

Analysis of Rock Mechanical Characteristics and Establishment of an Evaluation System for Coal Measures in the Longtan Formation, Guizhou Province

“…The fine sandstone, characterized by a P-wave velocity range of 3.07 to 3.70 km/s, has an average Pwave of 3.35 km/s. The average densities of the mudstone and silty mudstone are 2.75 and 2.79 g/cm 3 , respectively, while the average densities of the siltstone and argillaceous siltstone are 2.85 and 2.84 g/cm 3 , respectively. The average density of fine sandstone is 2.86 g/cm 3 .…”

“…The average densities of the mudstone and silty mudstone are 2.75 and 2.79 g/cm 3 , respectively, while the average densities of the siltstone and argillaceous siltstone are 2.85 and 2.84 g/cm 3 , respectively. The average density of fine sandstone is 2.86 g/cm 3 . The inherent presence of muddy bands within the mudstone strata, coupled with the greater development of pores and fissures compared to the sandstone strata, leads to the more discrete density and P-wave velocity of the mudstone, while the fine sandstone is dense and intact, so the changes in the P-wave velocity and density of the fine sandstone samples are small.…”

“…The natural potential of coal measure rocks is −23.46−3.99 MV, the natural gamma is 30.13−225.86 API, the acoustic time difference is 194.00−492.99 μm/s, the compensation neutron is 7.85−89.03%, the compensation density is 1.75− 2.78 g/cm 3 , and the deep lateral resistivity is 8.40−1993.00 Ω• m. Analyzing these logging parameters reveals characteristics associated with different rock types within the Longtan Formation. According to the main range of the various logging parameters, fine sandstone and siltstone typically exhibit medium to low natural potential and natural gamma values, along with medium to low acoustic time differences and compensated neutron values.…”

“…Coal measures serve as important hydrocarbon source rocks and reservoirs for coal measure gas (coalbed methane, tight sandstone gas, and shale gas, which coexist and can accumulate into valuable resources). − Currently, the exploration and development of coal measure gas remain in their nascent stages, and Guizhou Province is emerging as a pivotal coal measure gas exploration and development reserve region within China. This province boasts substantial geological coalbed methane reserves amounting to 3.15 × 10 12 m 3 , accompanied by a formidable 4.55 × 10 12 m 3 of Permian and Carboniferous coal measure shale gas resources, signifying the extensive developmental potential of coal measure gas. − Within the process of coal measure gas exploration and development, understanding the mechanical and fracturing characteristics of coal measure rocks and establishing a comprehensive evaluation system of rock masses applicable to coal measures are conducive to selecting suitable construction programs and parameters in the process of coal measure gas development (reservoir fracturing and reforming as well as combined layer drainage and mining), which will foster the efficient development of coal measure gas.…”

Petrophysical and Geochemical Investigation-Based Methodology for Analysis of the Multilithology of the Permian Longtan Formation in Southeastern Sichuan Basin, SW China

“…To achieve energy security and sustainability, it becomes imperative for China to enhance domestic gas production capabilities while simultaneously exploring alternative energy sources and implementing strategies to reduce reliance on imports. In 2021, China's coal production reached 4.13 billion tons, accounting for more than 50% of global coal production [9]. China's coal measure gas production is expected to exceed 1000 × 10 8 m 3 in 2030 [8].…”