Bohai Bay basin, in eastern China, is rich in oil resources, but recently large natural gas accumulations have been found, mainly in Miocene reservoirs, at depths shallower than 1500 m. Molecular and stable carbon isotopic gas compositions show that the gases have several distinctive biogenic origins. The first type of biogenic gas is generated in anaerobic environments from thermally immature organic matter and this type of gas accumulates typically in reservoirs interbedded within the immature source rock succession. The second type occurs up dip or above heavy-oil reservoirs and the gas is inferred derived from crude oils in shallow reservoirs as a biodegradation product. The third type originates from source rocks that were buried deeply previously (>2800 m in the basin), where they were “geopasteurized”, but which are now uplifted and recolonized by microbes at their current shallow depths. The first type is generally referred to as primary biogenic gas. The second type is commonly referred to as secondary biogenic gases elsewhere, but herein is called biogenic gas from biodegraded oil (BG-BO). This distinguishes them from the third type which is described herein as secondary biogenic gas and which is distinguished from BG-BO. Bohai Bay Basin gas pools often contain one of the three biogenic gas types and are often mixed, more or less, with thermogenic natural gas from petroleum pools in Eocene reservoirs. It is therefore important to distinguish the genetic affinities of the biogenic natural gases as an aid to the development and exploration for additional biogenic gas resources in Bohai Bay Basin.

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