Different distribution and origin of hydrocarbon properties of coal-bearing rock series—Case study in J1b1 Formation in the slope area of Mahu Sag

Key words: 
• Key words: Mahu slope region /
• J1b1 Formation /
• J/T unconformity surface /
• Compaction-centrifugal flow /
• Coal seam desorption /
• Carbon isotope

Abstract: Abstract: J1b1 Formation in the Mahu slope area is a set of sandy (gravel) rock deposits of coal-bearing rock series close to the J/T unconformity. The shale rich conglomerate belt near the bottom of the J/T unconformity has obvious oil and gas thickened, and light oil and gas are mainly concentrated in the thin sandstone belt above the coal seam. There is still no research on the origin of oil and gas thickening zone in relatively deep buried area, the percolation mechanism of atmospheric fresh water to overlying strata on unconformity surface, and the origin of light oil and gas enrichment above coal seam of coal-bearing rock series. Therefore, based on the properties of crude oil and carbon isotope analysis of natural gas, combined with the data of burial history, thermal history, pore evolution history, and laser Raman spectrum analysis of hydrocarbon salt water inclusions, the genetic types of J1b1 natural gas and the origin and distribution of hydrocarbon thickening zone and light oil and gas zone are discussed. The results show that: ①J1b1 coal-bearing rock series can be divided into oil and gas thickening zone and light oil and gas zone according to the characteristics of oil and gas density viscosity. The former is mainly distributed in the upper wall of the fault zone, the fault zone, and the bottom of the slope zone adjacent to the J/T unconformity. The latter is mainly distributed in the thin sandstone zone above the coal seam in the slope zone, and the coal-derived gas content is high. ②The origin of the shale rich conglomerate oil and gas thickening zone at the bottom of the slope zone adjacent to the J/T unconformity is closely related to the atmospheric freshwater compacting-centrifugal flow oxidation degradation. The “YANSHAN normal fault-J /T unconformity -shale rich conglomerate assemblage”and the associated capillary imbibition of wetting water phase led to the formation of atmospheric freshwater compaction-centrifugal flow oil-gas thickening zone closely related to the unconformity in the slope area of relatively deep burial. ③The thin sandstone reservoir above the coal seam has the characteristics of “self-generated and self-stored (deinspiratory charging in J1b1 coal seam)” and “bottom-generated and upper-stored (hydrocarbon supply from P1f source rock)”, and has the advantage of light oil and gas enrichment. The coal seam desorption diffused upward through the positive fracture into the thin sandstone at the top, and the methane carbon isotope value was relatively light, which was different from the retained adsorbed gas in the coal seam (δ13C1=-48.2‰ in the former and δ13C1=-43.6‰ in the latter). The geological stratification effect caused by long distance migration of P1f source rock oil and gas further promotes the accumulation of light oil and gas in thin sandstone above the coal seam. The conclusion is that the upper thin sandstone belt of J1b1 is the first choice for exploration of light oil and gas enrichment belt. The quantitative lithological logging identification map of coal-bearing rock series with the combined restraint of CNL-IMP was established, and the effective reservoir distribution area of the upper thin sandstone of J1b1 with a thickness≥10m was identified in the Yanbei1 3D seismic zone.