The Identification and Implication of Injectites in the Shwe Gas Field, Offshore Northwestern Myanmar

Cossey, Stephen; Kim, Dongil (David); Yang, Su-Yeong; Jung, Ho Young

A large complex of sedimentary injection features (injectites) has been identified in the Pliocene offshore Shwe biogenic gas field in Myanmar. The injectites provide vertical and lateral continuity from a basal proximal lobe sequence (G5) to an overlying distal lobe sequence (G3). The G5 has a unique petrographic signature (low rock fragments, high mica) and the G3 shows low mica and high rock fragments. The injectites were discovered in one of the cored wells because there were anomalous structureless sands in the G3 which showed the petrographic characteristics of the underlying G5. The sills range in thickness from 5 cm to 2 meters thick in cored wells, have sharp bases and tops and show symmetrical grain size trends either side of the middle of the bed. In uncored wells, they are interpreted to be up to 6 meters thick with uniform (characterless) gamma ray response. Once they were identified in core, several seismic scale features, such as wings, steeply dipping reflectors and an irregular G5 reservoir top were identified on the 3-D seismic. The injectites were probably triggered at least twice by several overlying slumps which rapidly buried and overpressured the sand-rich lobes of the G5. The gas-filled injectites seem to be mainly restricted to the northern part of the field where the underlying G5 reservor is anomalously thin. The discovery of the injectites drastically changed the reservoir model and explained many of the unusual features of the field such as a common gas gradient, perched water and isolated pressure cells.

AAPG Search and Discovery Article #90163©2013AAPG 2013 Annual Convention and Exhibition, Pittsburgh, Pennsylvania, May 19-22, 2013