VOLCANIC RESERVOIR ROCKS: A CASE STUDY OF THE CRETACEOUS FENGHUADIAN SUITE, HUANGHUA BASIN, EASTERN CHINA

The lithofacies and reservoir properties of Cretaceous volcanic rocks in the Fenghuadian Suite (Huanghua Basin, eastern China) are described in this paper. Three lithofacies have heen identified (pyroclaslics, lavas and volcaniclastics) and each has different reservoir potential due to differences in porosity and permeability. Pyroclastics and lavas have better reservoir properties than do volcaniclastic rocks. In the pyroclastics, voids and fractures within brecciated rocks together with dissolution voids and fissures result in highly connected porosity (up to 22%). In the lavas, vesicles, dissolution joints and cooling fissures together with weathering cracks and fissures are abundant and have high connectivity in the upper portions of each volcanic cycle, giving rise to excellent reservoir properties. By contrast, volcaniclastic rocks have low porosities and poor reservoir potential. We suggest that porosity development is partly related to palaeo-geomorphology. Thus, volcanic rocks inpalaeo-highs and in neighbouring transitional areas have enhanced reservoir properties as a result ofweathering and the consequent development ofvugs and fissures. Chromatographic analyses of crude oils and rock extracts indicate that the Kong-2 Member of the Eocene Kongdian Formation, which overlies the Fenghuadian Suite in the study areaandwhich also occurs in the nearby Cangdong Depression, is the principal source of hydrocarbons reservoired in the Fenghuadian Suite. Faults may have acted as conduits for hydrocarbon migration.