Diagenesis in Paleogene Intermontane Clastic Reservoirs, Qaidam Basin, Western People's Republic of China: ABSTRACT

Abstract

Oil and gas are produced from diagenetically enhanced reservoirs in nonmarine Paleogene clastic rocks in the Qaidam (Chaidamu) basin, western People's Republic of China. Production is now from fluvial sediment less than about 1000 m in depth, but recent drilling has revealed oil-stained, lower Oligocene siltstones and sandstones up to 5 m thick at depths greater than 3500 m. Limited data indicate that the oil-stained sediments, deposited in a nearshore environment, are laterally nonpersistent, relatively fine grained, and poorly sorted. Examinations of thin sections and core chips obtained from a drill hole in the southwestern part of the basin indicate that porosity is highly variable, ranging from zero to about 20%. Limited data indicate that porosity decreases with increasing depth, but not in a regular or generally predictable manner. Although there is evidence of porosity loss due to compaction of labile rock fragments, the dominant control of porosity is the presence or absence of calcite and/or anhydrite cements. Leaching of the early formed cements has produced secondary porosity and has increased storage capacity in the present reservoir rocks. There is no clear indication that the presence (or quantities) of cements are depth related, nor is there sufficient evidence to relatemore » the cements to depositional environments.« less