| dc.description.abstract | A model has been established to examine the hydrocarbon kitchen and migration routes of the Tiehchenshan gas field (TCS), which illustrates why an anomalous quantity of hydrocarbon was accumulated in TCS, even though the structure was formed quite late. During the late Oligocene to the Pliocene’s Chinshui stage, the Taishi basin accumulated sedimentary deposits over 4500 m with the center kept at the Hinchu-Miaoli area. The Mushan formation could be an oil-bearing layer, which generated large amounts of hydrocarbon migrating along the up-dip direction to the main reservoir. The new tectonic movement reloaded the depocenter to the southeast of Taichung-Changhwa area, which changed the hydrocarbon trap in the pre-loaded structures, but without much influence on the pattern of hydrocarbon kitchen.
The GOR values have been used to investigate the direction of hydrocarbon migration. A trend of GOR variations with time was derived from the long-term GOR distribution by a least square fitting approach. The residual GOR values deduced after removing the trend can be used as an indicator of hydrocarbon migration. The distribution maps of initial GOR and residual GOR reveal migration patterns of the TCS and the Chinshui-Yunghoshan (CS-YHS) gas fields. These two gas fields possess a single production layer with regular trends of GOR distribution. Sandstone beds of the middle Miocene Talu Shale are the main gas reservoirs. The regular spatial GOR distribution pointed out that the direction of lateral hydrocarbon migration is from east to west for TCS and NW to SE for YHS, respectively.
During the Chinshui stage, much hydrocarbon might have been accumulated in the foothill of Hinchiu-Miaoli area. The hydrocarbon is inferred to migrate to the position of the present Tunglo syncline and its northern extension before thrusting-and-folding occurred in this region. When the new tectonic movement starts, the fault system was reactivated and formed migration paths, a great deal of hydrocarbon migrated into the TCS structure. The porosity and permeability of formations of Peiliao, Chuhuangkeng, Piling, Mushan and Wuchihshan are low, which interrupt the migration of hydrocarbon into these layers. Instead, the sandstone beds of Talu Shale have high porosity and permeability values, thus become the gas reservoir of the TCS gas field. This model strongly suggests that good opportunity of new gas fields may exist to the east of TCS, which is worth
further exploration.
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