Abstract

The Chum Saeng Formation of the Phitsanulok Basin, onshore Thailand, is an Early Miocene organic-rich lacustrine shale that provides good source rock and seal to existing conventional hydrocarbon accumulations. The Phitsanulok Basin is the largest onshore Tertiary rift basin and also the largest oil production for onshore Thailand. The basin is situated in the Lower part of the Northern Thailand, approximately 400 km north of Bangkok. The field was discovered in 1981 by ThaiShell and was put on production since 1983. The main hydrocarbon production is from structurally trapped fluvial
dominated lacustrine deltas of the Lan Krabu Formation and fluvial dominated Yom and Pratu Tao formations. Average thickness of the Chum Saeng Formation ranges from 50 to 400 m, with total organic carbon (TOC) ranging from 5 to 20 wt% There currently is no unconventional production from the Chum Saeng Formation. However, the Chum Saeng Formation is hypothesized to contain pervasive (also known as basin-center accumulation, continuous accumulation, and deep basin) oil and gas accumulations. Pervasive accumulation describes the extensive unconventional hydrocarbon
accumulations within a basin. Most pervasive accumulations are abnormally pressured (high or low), commonly lack a downdip water contact, and have low-permeability reservoir rocks. This study investigated the potential unconventional petroleum system of the Chum Saeng Formation. Petroleum system elements and processes were. used as tools to assess the petroleum potential of the Chum Saeng Formation. The subsurface work done included: (1) mapping thickness, (2) mapping organic richness, (3) mapping thermal mahlrity, (4) identifying the hydrocarbon generation window, (5) interpreting source rock depositional environment, (6) source rock analysis, (7) one-dimensional basin modeling, (8) analysis of formation pressure, and (9) analysis of significant gas shows from mud log. Producing oil and gas fields from the Greater Green River, Uinta, and Piceance basins in the Western United States were used as analogs. The conventional petroleum within the Phitsanulok Basin results from hydrocarbon migration from the thermally mature part of the basin ("kitchen") into structural traps. These accumulations occur updip and around the hydrocarbon generation window and feature normal to slightly overpressured. The conventional reservoir rocks are sandstones with good porosity and permeability. Pervasive oil and gas accumulations are identified in the center of the basin, the Sukhothai Depression, within both the oil and gas generation windows. Potential reservoir rocks occur within the Chum Saeng (source), Sarabop, Lan Krabu, Pratu Tao, and Yom formations, range from poor (less than 10 %porosity) to good (more than /0 %porosity) reservoir quality. The Chum Saeng Formation shares many similar properties to those of the currently producing pervasive oil and gas fields in the Greater Green River, Uinta and Piceance basins. However the rock units in the Phitsanulok Basin have lower pressures and reservoir rocks are expected to be less fractured, so there is a very low chance the rock will produce naturally; also the reservoir continuity is low. Reservoir enhancement (porosity and permeability) by hydraulic fracturing will play a crucial role in developing these potential pervasive accumulations. The study result provides new exploration opportunity to the
matured basin, by applying the unconventional thinking and using existing producing fields as analogs. Further investigations are including drilling unconventional exploration well to get the direct formation information.

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