Print this pageHydrocarbon Charge History and Paleo Oil Reservoir Restoration in Anyue Cambrian Gas Field in Sichuan Basin: Detailed Evidence From Fluid Inclusions and Quantitative Grain Fluorescence
Abstract
Anyue gas field in the Longwangmiao Formation (L. Fm) of Cambrian, located in the Moxi area of the Sichuan basin, is the largest mono-block carbonate gas field in China, with proved gas reserve exceeding 4400×108 m3. Gas composition, light hydrocarbon and reservoir bitumen content have evidenced that the gas is dominantly sourced from crude oil cracking gas. The formation process of gas reservoir is complex, and is closely related to the palaeo oil reservoirs. Investigation on palaeo oil reservoirs distribution and hydrocarbon charge history could provide more information for understanding deeply gas accumulation in L. Fm and guiding exploration in future. In this study, a comprehensive fluid study was performed including fluid inclusion analysis(Parnell et al., 1998; Kerkhof et al., 2001), quantitative grain fluorescence(Liu et al., 2013) and Raman spectrum measurement. In L. Fm, minerals filling sequences in the fracture and karst cavity are predominately from dolomite to solid bitumen, quartz + dolomite. The carbonate rocks contain both hydrocarbon and aqueous inclusion. Most of the fluid inclusions are hosted within just two minerals: dolomites and quartzes. Three types of fluid inclusions are recognised as determined by petrography and microthermometry. Oil and aqueous inclusions within dolomites and clear edge of dolomite with homogenization temperature (Th) range of 100-120 °C indicates the primary oil charge and accumulation. Quantitative grain fluorescence results shows the oil has a low density. Bitumen, gas-aqueous two phase and aqueous inclusions formed in dolomite vein have a range of 130-170 °C in Th. That provides an evidence for the secondary hydrocarbon charge during the beginning of oil thermal cracking in the period of late Triassic epoch. Methane and gas-aqueous inclusions within quartz vein have a high Th range of 200-230 °C. It is the evidence for the third gas charge and accumulation in the period of Jurassic and Cretaceous. The gas predominately sourced from a large number of oil thermal cracking. Present gas reservoirs were finally formed during the period. According to the fluid inclusions analyses, quantitative grain fluorescence and bitumen content measurement, a giant palaeo oil reservoir in L.Fm was restored, which verifies that Anyue Cambrian gas reservoir is a secondary pools with oil cracked in situ.
AAPG Datapages/Search and Discovery Article #90350 © 2019 AAPG Annual Convention and Exhibition, San Antonio, Texas, May 19-22, 2019