Geological Framework and Gas Hydrate Potential in Deepwater Fold-and-Thrusted Belt of the Lower Gaoping Slope Offshore SW Taiwan

Abstract

Abstract

Gaoping slope in offshore southwestern Taiwan is an active margin formed due to the westward-propagation of the compressional field of Taiwan orogeny and Manila subduction System that accrete post-rift sediments on the northeastern slope of South China Sea (Chinese continental margin) to the orogenic wedge. This study focuses on the deepwater fold-and-thrust belt of the lower Gaoping slope which is located in the frontal convergent wedge. The post-rift sedimentary strata in deepwater area of the passive Chinese continental margin are separated into five stratigraphic units base on different seismic characteristics. These mapped units extend to the frontal part of the convergent wedge. An important observation is that the thickness of eastern Unit 5 (the latest unit) is thicker than that in the west of the study area, and is characterized by channel-levee features, suggesting that the sediment from Taiwan are transported through submarine canyons and channels to construct turbidite system in deepwater area. The folded turbidite deposits not only provide permeable strata for upward gas migration, but also become porous reservoirs for gas hydrate and free gas. Folded anticline structures play the role for gathering and trapping fluids beneath the structural high. In addition to dipping strata, the faults in this area can also be good fluid migration pathways for deep gas to move upward. In this area, several fluid-related signals are identified. Clear bottom simulating reflector (BSR), an acoustic indication of the base of gas hydrate stability zone, are broadly distributed in the lower Gaoping slope, especially beneath topographic highs. Bright seismic reflectors which show polarity reversal when crossing the BSR provide an evidence of free gas moving along the permeable strata to the gas hydrate stability zone to form gas hydrate. Possible flat spots are also identified in some places to show the gas water contacts. Low velocity zone beneath BSR indicates free gas accumulations there. Although there is no direct information on lithology, seismic stratigraphy analysis and numerous fluid-related signals in the lower Gaoping all suggest that a large amount of gas hydrate are present in the deepwater fold-and-thrust belt offshore SW Taiwan.

AAPG Datapages/Search and Discovery Article #90260 © 2016 AAPG/SEG International Conference & Exhibition, Cancun, Mexico, September 6-9, 2016