Mou, Duenchien¹, Fu-Chen Su², Jhy-Shing Ting³ 
(1) Exploration Consulting International, Bakersfield, CA
(2) Chinese Petroleum Corporation, Miaoli, Taiwan
(3) Chinese Petroleum Corporation, Taipei, Taiwan

ABSTRACT: Trapping Mechanism of the X-Structure Gas Accumulation, Offshore Taiwan: A New Look at An Old Find

Gas in the X-Structure, offshore Taiwan was first discovered many years ago. Although a number of appraisal wells were drilled and 3D seismic was acquired since then, the complexity in reservoir quality and compartmentalized nature of the accumulation hampered the decision to go into actual development. 
Recent reevaluation of the area has revealed a structural and stratigraphic combination trap for the accumulation. Gas sourced from the deeply buried Oligocene and Mesozoic in the Tainan basin at its northwest migrated up the NW-dipping flank of an old pre-Oligocene NE-trending horst block, along the Oligocene unconformity surface within Oligocene and Cretaceous sandstones above and below. Accumulations formed primarily in separate fault blocks within the subcropped and fractured Cretaceous and basal Oligocene sandstones, under an overall lateral seal made of impermeable Late Oligocene canyon-fill silt and clay facies package. Some of the gas leaked upward into younger Oligocene and Lower Miocene discontinuous turbidite sand bodies, forming additional accumulations. 
Using well and seismic data to differentiate Oligocene facies and reservoir pressure and fluid contact information to recognize separate compartmentalized gas pools has allowed a reliable picture of the gas distribution. Reserve estimate, development strategy, and future step-out locations for upside potential can all be deduced from the new understanding of the trapping mechanism for the X-Structure gas.

AAPG Search and Discovery Article #90026©2004 AAPG Annual Meeting, Dallas, Texas, April 18-21, 2004.