Natural gas washing induces condensate formation from coal measures in the Pinghu Slope Belt of the Xihu Depression, East China Sea Basin: Insights from fluid inclusion, geochemistry, and rock gold-tube pyrolysis

Abstract

A systematic study into the origin and potential formation processes of condensates in the Pinghu Slope Belt of the Xihu Depression, East China Sea Basin, was conducted using fluid inclusion analysis, petroleum geochemistry, and rock gold-tube pyrolysis. Grains containing Oil Inclusions (GOI™) technique, well logging, and formation testing together indicate that the present-day gas condensate layer has evolved from a paleo-oil layer. Biomarkers from oil inclusions indicate that these paleo-oils were generated from coal measures at a moderate maturity level. Based on organic macerals, elemental ratios of kerogens, Rock-Eval pyrolysis, and results from gold-tube pyrolysis experiments, the coal measures are evaluated as source rocks prone to gas generation and capable of oil formation at moderate levels of maturity. This maturity level coincides with present-day condensate maturities determined from a series of molecular maturity ratios (vitrinite reflectance ~ 0.7–1.0% Ro). Biomarkers related to specific sources suggest that the condensates are also coal-derived. These observations suggest that the condensates are derived from secondary alteration following paleo-oil accumulation, rather than being related to the thermal degradation of resin-rich organic matter at a low maturity level, or the thermal cracking of kerogen at a high maturity level.Some condensates are characterized by a significant loss of n-alkanes with low carbon numbers and relatively high toluene/n-heptane (Tol/nC7) ratios. This indicates that the paleo-oil reservoirs have been subjected to varying degrees of gas washing, which resulted in condensate formation, as opposed to the process of oil cracking under high temperatures. Subsequent interactions between gas and paleo-oils resulted in oil inclusions showing fluorescence varying from near-yellow to blue. Variable vapor volume fractions measured in the same oil assemblage are the result of two-phase trapping of oil and gas. The widely observed presence of bitumen (asphaltene) in the reservoir pores and bitumen-bearing oil inclusions is considered to be due to gas deasphalting.Homogenization temperatures (Th) of associated aqueous inclusions and burial-thermal curves suggest that the initial paleo-oil reservoirs formed in the late Miocene. Excess gas, composed of local mature gas and heterochthonous high-to over-mature gas, migrated into the paleo-oil reservoirs later, in the Quaternary. This process not only resulted in the formation of condensates, but also led to vertical differentiation of density and wax content in the reservoirs.