Abstract

Hydrocarbon reservoirs in the Oulituozi area of the Liaohe basin, northeastern China, occur within a subvolcanic trachyte porphyry intrusion. This intrusion was emplaced at a depth of less than 1 km during the Paleogene. The prevalent reservoir porosity is in explosive breccias, fracture networks, steeply dipping cracks, and dilational microfractures in the apical zone of the intrusion. These voids were formed by underground explosion during magma solidification and by postsolidification volume contraction. Minor oil and gas volumes are trapped also in fractures in the overlying brittle beds. The massive rocks in the lower part of the porphyry body serve as hydrocarbon seals for reservoir beds lateral to the intrusion. We propose a model for reservoir formation related to a subvolcanic intrusion that includes three types of reservoirs: (1) reservoirs in the apical zone of the intrusion, (2) reservoirs in the upper wall of the intrusion, and (3) reservoirs lateral to the intrusion. Attention should be paid to all the three reservoir types during exploration around a subvolcanic intrusion.

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