MIGRATION OF OILS INTO SAMAAN FIELD, OFFSHORE TRINIDAD, WEST INDIES

PHILLIP D. HEPPARD
Amoco Production Company
501 Westlake Park Blvd.
Houston, Texas 77079, U.S.A.
ROGER L. AMES,
LARRY M. ROSS,
Amoco Production Company
Amoco Tulsa Research Center
4502 East 41st Street
Tulsa, Oklahoma 74135, U.S.A.

The Samaan Field is operated by Amoco Trinidad Oil Company and lies 22 miles (35.4 kin) off the southeast coast of Trinidad. The field has produced 184 million barrels of oil. The hydrocarbons are trapped in an anticlinal structure which is an offshore extension of the petroliferous Southern Range of onshore Trinidad. The structure formed during late Pleistocene deformation. The anticline is complexly cut by normal faults which formed contemporaneously with the deformation. The reservoirs are very fine grained sands and sandstones separated by sandy shales (mudstones). The deepest wells to 12,000 feet (3660 m) penetrated an abnormally pressured section. Within the field, the molecular composition of the oils show great variability. These oils have migrated from deeper source rocks of Cretaceous age and are now trapped in reservoir sands and sandstones of Pleistocene and Pilocene age. The oils exhibit a systematic stratification from waxy paraffinic crudes with lower API gravity in deep sands to nonwaxy paraffinic crudes with higher API gravity in the shallower sands. The compositional variations in the reservoirs are due to secondary migration effects and fractional devolatillzation and, to lesser degrees, water-washing and biodegradation. The molecular differences attributed to migration can be separated from those due to the other alteration mechanisms and provide clues to migration pathways. Normal faults with throws of 200 to 800 feet (61-244m) separate equivalent reservoir sands containing oils that exhibit compositional differences due to secondary migration and separate migration pathways. The conclusion from this study is that the faults controlled migration and pre-date the movement of oil into the field. Five recent wells from Samaan platforms have discovered new reserves based in part on the application of this geochemical interpretation.

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