Origin of eyeball-shaped limestone and its significance for petroleum geology

Eyeball-shaped limestone is a kind of carbonate rock with a structure shaped as eyeball and eyelid, which is widely developed in south China. The petrological characteristics, paleo-sedimentary environment, origin and hydrocarbon geological significance of eyeball-shaped limestone were analyzed and discussed through a great deal of review and investigation on the characteristics and origin of domestic and abroad eyeball-shaped limestone. The eyeball-shaped limestone is composed of the "eyeball" of micrite-silty bioclastic limestone and the "eyelid" of argillaceous limestone or calcareous mudstone. Its origin mainly includes sedimentation and diagenesis. The "eyelid" of eyeball-shaped limestone is high-maturity to over-mature source rock with medium organic matter content deposited in deep water environment, which has good hydrocarbon generation potential. The eyeball-shaped limestone is also a set of low-porosity and low-permeability reservoir, and the storage performance of "eyelid" is obviously better than that of "eyeball". Fractures can improve the reservoir physical properties of eyeball-shaped limestone, and the reservoir with fractures has good oil and gas display.On the whole, the eyeball-shaped limestone gas reservoir is a self-generating and self-storing carbonate gas reservoir.