Wide occurrence of leopard-pattern limestones (dolostone) has aroused extensive attention of scholars

but origin of this type of rock remains disputable.So a study on its features

origin

and impact on reservoir properties is significant to petroleum exploration.Four reference cross sections were chosen in the Lower Cambrian Longwangmiao Formation and Tianheban Formation in the Sichuan Basin to analyze petrologic and geochemistry characteristics.Based on outcrop sedimentary facies study

microscopic observation

X-ray diffraction analysis

scanning electron microscopic observation

electron probe microanalysis

and stable isotope analysis

the origin of the limestone was discussed.Leopard pattern limestone occurs mainly in two types of rocks in the Lower Cambrian sediments in the Sichuan Basin:wackstone and packstone.According to their different features in patch morphology

clay content

dolomite size and crystal form

leopard pa-ttern limestone can be classified into three types:muddy leopard limestone

leopard limestone containing mud

and leopard limestone.The first two types mainly appear in wackstone

composed of muddy to silty

anhedral to semi-euhedral crystalline dolomite

while the third type of leopard pattern limestone occur in packstone

composed of silty to fine

semi-euhedral to euhedral crystalline dolomite.Leopard pattern limestone in wackstone deposited in subtidal environment with low water energy

lime mud and aragonite mud(transform to dolomite) deposited alternately in tune with the water body transform from open to restricted frequently due to sea level change.Leopard pattern are formed by wave disturbance and di-fferential load from overlying strata.Leopard pattern limestone in packstone was mainly formed during shallow burial stage

when formation water with high content of magnesium penetrated into the cemented sediments and dolomitization took place.Intercrystalline pore space is widely observed in leopard limestone

which shows much better reservoir properties than limestone

so the proportion of leopard pattern limestone to the whole rock determines the petrophysical properties of reservoir rocks.