A High Resolution Sequence Stratigraphic Approach to Correlate Complex Sub-Seismic Tidally-Influenced Estuarine Incised-Valley Fill Reservoirs of the Lakshmi Field, Gulf of Cambay, India

Sanyal, Satyashis¹; Wood, Lesli²; Chatterjee, Dibyendu¹; Dwivedi, Nikhilesh¹; Burley, Stuart¹
¹Cairn India, Gurgaon, India.
²Bureau of Economic Geology, University of Texas at Austin, Austin, TX.

The Lower Tarkeshwar reservoir interval in the Lakshmi Field, Gulf of Cambay, Western offshore India was deposited in a tidally-influenced estuarine valley fill setting during Late Oligocene to Early Miocene times. High resolution electro-log based sequence stratigraphic correlation suggests that the reservoir sands are deposited within 3rd and 4th order cycles, resulting in a series of coarsening-up and fining-up para-sequences under an overall transgressive global sea-level period. Major phases of shallowing-up identified in the stratigraphic framework enabled the recognition of the log motifs in the reservoir interval comprising of stacked confined tidal bars, unconfined and isolated tidal bars, broad tidal sand sheets, tidal flats comprising of tidal creeks and smaller tidal channels, proximal stacked prograding sand bars and open estuarine background mud. Cross-bed dips derived from image logs supported with inferred sand thickness trends from seismic attribute analysis suggests a major fairway of deposition trending parallel to the long axis of the structure.

The high-resolution log motif based facies classification correlated across the dip of the field indicates that the reservoir interval comprises at least 3 phases of episodic deposition within the incised valleys. The basal fill is dominantly down-dip and consists of stacked low-stand or early transgressive distributary channel and confined bar facies, which occur in an overall fining-up succession, with each smaller cycle of progradation reflecting the development of a bar complex. The second episode of valley fill represents more unconfined tidal bar development grading laterally and upward into a dominantly tidal flat and creek system. These are capped in the downstream direction by broad transgressive sand sheets, with excellent lateral continuity and reservoir properties. The late stage valley fills are characterized by increased deepening into isolated reworked tidal bars within broad tidal flats, with a significant increase in mud content before a major erosion event marks the end of the incised valley filling.

This high resolution facies correlation approach provided a revolutionary improvement in deciphering the reservoir architectural elements and understanding the lateral and vertical connectivity of individual sand bodies, aspects of which are critical for an optimized field development in complex depositional environments.

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