Upside-Down Sequence Stratigraphy, Sandy Highstands and Muddy Prograding Complexes in the Surma Basin, Bangladesh

Barbara J. Radovich, Michael W. Hoffman, Martin A. Perlmutter

Several large, TCF-size gas fields have been discovered in the Surma Basin, Bangladesh. Detailed sequence stratigraphy was performed on log and seismic data to study these fields and future potential of the area. The prospective section is Upper Miocene sands caught up in a series of younger compressional fault-related folds. World-class gas/water contacts are observed on the seismic data over the fields. Previous workers have also recognized deep incised channel cuts on the seismic data. Sequence stratigraphic techniques applied to log and seismic data reveal a cyclic stack of sandy, shallow shelf transgressive and highstand systems tracts, with muddy, silty marginal marine lowstand wedge sediments filling in large incisions formed at each sequence boundary. This section offers multiple reservoir/seal couplets, with significant gas pays sealed either by mudstones of the lowstand wedge in proximal environments or by thin condensed sections overlying transgressive sands.

This Surma Basin section represents a continuous aggradation of a series of Upper Miocene sequences with lithology distributions that are not commonly seen in passive margin settings. This stratigraphic position is updip from the usual Gulf of Mexico section where highstands are generally thin and shaly and the prograding complexes are expanded and full of sand. Regional seismic lines from the Surma Basin show the characteristic prograding reflection geometries of the prograding complex develop much further basinward to the south. Prominent, regionally mappable transgressive surfaces overlay these units.

The muddy, aggraded prograding complexes may be an excellent example of reduced sedimentation rates during deposition of the prograding complex to transgressive systems tract. This situation has been suggested by sedimentary models which incorporate climate change and sea level change. The lithology distributions in the Surma Basin best fit a model where the sediment supply cycle is out of phase to the sea level cycle.

AAPG Search and Discovery Article #91020©1995 AAPG Annual Convention, Houston, Texas, May 5-8, 1995