Depositional Environment, Thin Bed Potential, Planned Evaluation at Sangu field, Bay of Bengal, Bangladesh

Matesic, Marin
South East Asia Exploration, Santos, Oil and Gas company, Adelaide, SA, Australia.

The Sangu Field in the Bay of Bengal, Bangladesh produces gas from tens to hundreds of milidarcy sandstones of Pliocene-age. Santos is planning to drill two to three wells in the Bay of Bengal area in 4Q 2013.

The depositional environment of the reservoir rocks is very similar to the modern depositional setting in the Bay of Bengal. Two distinctively different reservoir rock "packages" are observed: hetherolitics (interbedded sands and shales) and massive clean sandstone beds.

Integrated sedimentological and ichnological analysis of seven cored wells Sangu 1, 2, 3, 3Z , 4, 5 and Semutang 5 by Gowland and Taylor 1998, has defined 10 facies associations based on lithofacies and ichnofabrics. Most of the facies association represent sediments developed in a typical siliciclastic tidal flat and comprise: intertidal mixed flat, subtidal mud flat, subtidal mixed flat, subtidal sand flat, tidal channel, tidal sand shoal (two types), offshore transition zone, shelfal muds. They were deposited as hetherolitic units and are thinly bedded.

The massive fine grained sandstone in the section is interpreted as having been deposited by high density turbidity currents. A deep water origin would not be consistent with the associated tidal deposits, raising the possibility that the turbidites occupy a shelfal setting and were sourced directly from major distributaries during peak discharge. Such an event could be attributed to the monsoon season, when massive volumes of suspended sand load can evolve into concentrated sediment flows dispersed across the shelf. After such flows terminated, the newly formed sediment substrate was available for reworking by tidal currents. Thus, the concept of suspended load dominated flows is proposed (hyperpycnal flow by Bates 1953).

The massive fine grained sandstones, deposited from high density currents have proven to be a good gas producer. Individual sands with relatively small area and thickness are capable of producing significant amount of gas. However, the hetherolitic, thinly bedded sands are the main producers in Sangu field. The minimum limit of producing sand thickness is poorly understood. Besides bed geometry, the high clay mineral content affects the conventional log interpretation.

Two significant issues have to be addressed in the future drilling campaign. Firstly, better understanding of massive sand distribution and calibration of sand geometries interpreted on seismic is required. Secondly, any potentially bypassed thinly bedded pay in the tidal-dominated delta sands should be appropriately characterized. For that reason use of Image Logs, Nuclear Magnetic Resonance, Wireline Pressure Tester and Rotary Sidewall Coring has been proposed.

AAPG Search and Discovery Article #90155©2012 AAPG International Conference & Exhibition, Singapore, 16-19 September 2012