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Abstract

AAPG Bulletin, V. 99, No. 3 (March 2015), P. 555ndash583.

Copyright copy2015. The American Association of Petroleum Geologists. All rights reserved.

DOI: 10.1306/09181413165

The dynamic behavior of shallow marine reservoirs: Insights from the Pliocene of offshore North Trinidad

Nigel E. Cross,1 Zana K. Williams,2 Arman Jamankulov,3 Candice E. Bostic,4 Valini C. Gayadeen,5 Helisaul J. Torrealba,6 and Elizabeth S. Drayton7

1BG Trinidad and Tobago Limited, BG House, 5 St Clair Avenue, Port of Spain, Trinidad and Tobago; present address: Mubadala Petroleum, Level 27, Al Maqam Tower, Sowwah Square, Al Maryah Island, P.O. Box 4887, Abu Dhabi, UAE; [email protected]
2BG Group, 100 Thames Valley Park Drive, Reading, Berkshire, RG6 1PT, United Kingdom; present address: QGC, 275 George Street, Brisbane, Queensland, 4000, Australia; [email protected]
3BG Trinidad and Tobago Limited, BG House, 5 St Clair Avenue, Port of Spain, Trinidad and Tobago; [email protected]
4BG Trinidad and Tobago Limited, BG House, 5 St Clair Avenue, Port of Spain, Trinidad and Tobago; [email protected]
5BG Trinidad and Tobago Limited, BG House, 5 St Clair Avenue, Port of Spain, Trinidad and Tobago; [email protected]
6BG Trinidad and Tobago Limited, BG House, 5 St Clair Avenue, Port of Spain, Trinidad and Tobago; [email protected]
7BG Trinidad and Tobago Limited, BG House, 5 St Clair Avenue, Port of Spain, Trinidad and Tobago; [email protected]

ABSTRACT

The Pliocene reservoirs of the North Coast Marine Area (block NCMA-1) comprise shallow marine shoreface to shelf sandstones (up to 30 m [98 ft] thick) and interbedded shelfal mudstones organized into stacked upward-coarsening parasequences (ca. 25–70 m BLTN13165eq1 thick). Production behavior is strongly conditioned by the interplay of subregional sand distribution and aquifer connectivity, and intrafield reservoir architecture complexity, commonly in the form of clinoforms.

The M2 parasequence is a well-connected reservoir unit and forms part of a late highstand systems tract with a strongly progradational architecture. Large-scale reservoir connectivity can be demonstrated by pressure communication in producing fields up to 10 km (6 mi) apart. The widespread nature of the sand also links the reservoir to a significant aquifer that provides pressure support during field depletion. In contrast, clinoforms that dominate the M2 reservoir architecture at a variety of scales reduce lateral connectivity. Wells situated in individual sand bodies show significant pressure interference during production, whereas wells that straddle bounding clinoforms (possible parasequence boundaries), although in pressure communication, show more subtle pressure interference. In the Chaconia field, clinoforms that bound lobate sand bodies in the M2 reservoir have also apparently slowed the onset of water cut within the field by several years and therefore increased reserves recovery. Similarly, in the M6 reservoir of the Ixora field a significant bounding clinoform has partitioned the field through much of its production life, but production data are now suggesting breakdown and late-stage pressure support across the same clinoform. The M6 parasequences form part of a lower quality early highstand systems tract and are more discontinuous than their M2 counterpart due to well-developed bounding clinoforms. As such they are not characterized by large active aquifers and do not benefit from such prominent long-term pressure support. The M4 reservoir is an isolated sand-body complex that forms part of a high-frequency lowstand complex. It shows similar stratigraphic heterogeneity to that displayed in the M2, with variations in pressure decline exhibited within individual wells of the same reservoir unit. Its isolated geometry means it is not connected to a regional aquifer and is therefore undergoing volumetric decline with no water breakthrough.

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