Development of a Depositional Model from Facies Analysis using Petrophysical Logs and Drill Cuttings, NE Bay of Bengal, South East Asia
Vanessa Da Silva
Master Sci - 2008
Australian School of Petroleum
University of Adelaide
The stratigraphy and petrology of the northeast Bay of Bengal was studied using data obtained from fifteen well logs, regional 2D seismic, and cuttings from two wells (Hatia and Magnama). The character of this region is primarily influenced by the proximity of the Bengal Delta. This is the largest tidally-influenced deltaic system in the world. This study was performed to evaluate possible environments of deposition for the north-east Bay of Bengal.
The correlation of maximum flooding surfaces, using well logs, was performed to gain an understanding of the cyclicity of sea level variations, climate and sedimentary processes that have affected this region. Interpretation of seismic data provided information about the geometry of these sequences and their relation to each other. It was also used to provide information related to the lithology of the stratal packages. The mineralogy, petrology and biogenic content of the cuttings were examined using petrographic analysis, scanning electron microscopy, x-ray diffraction and palynological studies. These procedures were undertaken to provide a thorough understanding of the environmental factors affecting sedimentation.
Seismic data and well log correlation indicated that the area of interest was highly incised, resulting in very little continuity of stratigraphic intervals. The results of the mineralogical tests indicated a presence of dominantly kaolinite and illite, with relatively lower percentages of smectite and chlorite. Palynological analysis showed exiguous spore pollen matter. The high spatial density of incision and large-scale channelization indicate an area that was highly affected by sea-level fluctuations and significant erosion. The interbedded shale and sandstone type of lithology, along with the presence of larger clay minerals like kaolinite and illite, suggest that the zone of deposition was fairly near shore and was most likely the inner shelf zone. The sparse bioclastic content indicates a high-stress area of deposition, where organisms were unlikely to settle for long periods of time. Deposition took place during periods of transgression followed by extensive periods of forced regression, most likely caused by glacio-eustatic events affecting the basin. This forced regression led to the cutting of the numerous incised valleys and canyons observed in the logs and seismic, while transgressive periods cause the infilling of these incisions.