Evolution of sedimentary deposition and assessment of petroleum potential of the Wichian Buri sub-basin, Changwat Phetchabun

Abstract

This research has the objective to define the subsurface geology of Tertiary deposits of the Wichian Buri sub-basin in terms of lithostratigraphy and tectono-sedimentation, with additional attempts to assess the petroleum potential of source rocks within the study area. The Tertiary sedimentary sequence in the Wichian Buri sub-basin is entirely non-marine sediments of lacustrine and fluvial deposits with the maximum thickness of 2,500 m. The proposed lithostratigraphy of Tertiary sequence of the basin has been subdivided into 4 formations, namely; WB-1 WB-2, WB-3, and WB-4 Formations in ascending order. The tectono-sedimentary evolution of the basin was dominantly by two extensional phases during Oligocene-Recent. The early phase is characterized by major normal faulting leading to basin fill, and igneous intrusions during Oligocene to Middle Miocene. Displacement along the border faults led to asymmetric half-garben and was the major driving mechanism for the subsidence during this time. The oldest basin fill which deposited during the initial stage of basin development in Late Oligocene, was represented by the fluvial sediments associated with rift volcanic of the WB-1 Formation. At the beginning of Early Miocene, increasing rate of extension with rapid subsidence led to open lacustrine conditions were established over the basin, resulting thick basin fill of dark, organic-rich claystone sequence of the WB-2 Formation. At the same time, fan deltas and alluvial fans continued to shed from the lake margins into the basin. During the Middle Miocene, an increasing fluvial influence occupied over the basin resulting from regional tectonic uplift that is in evidence of the presence of thick, cleaner, river sandstones of the WB-3 Formation. The second extensional phase of the basin began with thermal subsidence and widespread return of lacustrine conditions in the basin during Late Miocene to Pliocene. The renewed sedimentary cycle is characterized by a claystone dominant sequence of the WB-4 Formation that overlies on the Middle/Late Miocene unconformity. Following deposition, extension segmented the basin into rotated fault-blocks which were then covered by the Quaternary deposits after the Pliocene-Pleistocene uplift. Occurrence of petroleum in the Wichian Buri sub-basin has been clearly proven to be potential based on the discovery of oil and gas in the basin. Geochemical study of its sediments indicated that the dark organic-rich lacustrine claystone was likely to be the source of hydrocarbons discovered in the basin. It contained Type I/II kerogens with potential for generation of oil. It is found that, potential source rock in the basin are mostly immature except where it was affected by igneous intrusion that intruded the sedimentary sequence during Early to Middle Miocene.