Analyses of sedimentary

reservoir

and structural characteristics

were combined with application of such technologies as fluid inclusion and carbon and oxygen isotopes techniques

and observations of cores

thin sections and scanning electron microscope images

to study deposition during the Early Cretaceous Suhongtu stage in Chagan Sag.The result indicates a strike-slip and pull-apart faulting stage accompanied by multi-stage volcanic activities of thermal fluid flows dominated by neutral-basic andesite and subordinate basalt.The thermal fluid activities during the Suhongtu volcanic eruptions exerted significant influence upon layers deeper than 2 800 meters in the Bayingebi Formation and resulted in an abnormal high value of paleo-heat flow

an abnormal increase of homogenization temperature of inclusions

inorganic characteristics of carbon isotopes

an abrupt change in vitrinite reflectance

and an abnormal conversion of clay minerals.The thermal fluids from the deep layers carried gases (mainly CO

2

) and migrated upward along the Maoxi faults into the Wuliji tectonic belt.They then migrated horizontally along micro fissures and channels formed by unconformities to dissolve mostly carbonates and feldspar in the Bayingebi Formation.The signs of erosion were mainly found in places near deep-rooted large faults that controlled the migration of the fluids.Unconformities as well as secondary faults and fractures also played a part in controlling the flow of the thermal fluids.The dissolution was observed to be weakened toward the inner part of the sag.We therefore suggest that a study on the effect of deep thermal fluids activities upon the thermal evolution of organic matters and physical properties of reservoirs is the key to understand the distribution of dissolved pores and cavities in the study area and to explore for high-quality reservoirs there.