Control of Paleocene volcanic edifice on favorable reservoirs: a case study of the southwestern Huizhou Sag, Pearl River Mouth Basin

Abstract

Thickly developed Paleocene volcanic rocks in the southwestern Huizhou Sag of the Pearl River Mouth Basin are continuously filled with lacustrine sedimentary rocks of the overlying Eocene Wenchang Formation in the sag, forming the main rock formations in the early and middle stages of the rift basin. Absence of obvious sedimentary hiatus leads to poor supergene exposure as well as weathering and leaching effects of volcanic rocks. Therefore, the distribution of favorable reservoirs in volcanic rocks is mainly affected by volcanic edifices. Based on the latest drilling, logging and 3D seismic data, through the observation of cores and thin sections, the research on the volcanic edifice and lithofacies characteristics in the southwestern Huizhou Sag was carried out, and the controls of volcanic edifice on favorable reservoirs was analyzed. Results show that the study area has the progressive characteristics of "the eruption mode controls the type of volcanic edifice, the volcanic edifice controls the distribution of favorable facies, and the favorable facies controls the distribution of favorable reservoirs". (1) Controlled by the period and mode of volcanic eruption, stratovolcano and dome-like volcanic edifices developed in the study area, and the crater shape is of two types: subsidence caldera and dome. (2) The dominant lithofacies of volcanic rocks are distributed in the center of the volcanic edifice. The volcanic conduit facies, explosive facies and effusive facies in or near the crater are favorable combination of lithofacies. (3) The distribution of favorable reservoirs is affected by lithofacies assemblages and fault activities during the rifting period. The reservoir space is divided into primary and secondary pores and fractures. The lithofacies controls the enrichment degree of primary pores and fractures, while the later fault activities reform and connect the protogenetic pores and fractures, forming large-scale oil and gas reservoirs of pore-fracture type in volcanic rocks. (4) Extrusive facies, which is on the top of dome volcanic edifice, shows a poor oil-bearing property with hardly pores and fractures due to low volatiles content during the magma extruding.

Keywords

• paleocene
• favorable reservoirs of volcanic rocks
• volcanic facies
• volcanic edifice
• southwestern huizhou sag
• pearl river mouth basin