ABSTRACT: Development Geochemistry in Samaan Field, Offshore Trinidad

Roger L. Ames, P. D. Heppard

Oil geochemistry has been used extensively during the development of Samaan field off the southeast coast of Trinidad. Samaan field is an anticlinal structure producing from multiple Pliocene sandstones that are complexly cut by normal faults. The complex faulting makes structural mapping difficult despite well-developed sands, abundant well-bore control, and a three-dimensional seismic survey. Gas chromatography of wellhead samples provides an independent method to help define reservoir continuity and compartmentalization of the field.

Oils in Samaan field represent a single genetic type that has undergone significant vertical migration from deeply buried Upper Cretaceous or lower Tertiary sources. Initial sampling of all 34 producing wells in 1981 showed a wide range of molecular compositions that appear to be related to secondary migration. The most distinctive migration effect is the decrease of waxy (C₂₀₊) n-paraffins from deep to shallow sands. Isoprenoid/naphthalene ratios and n-C₁₇/pristane ratios also decrease systematically with decreasing reservoir depth.

At the 2 Sand level, the oils can be divided into three subclasses based on different C₁₅₊ n-paraffin distributions. The compositional subclasses indicate three compartments (e.g., central, eastern, and western) separated by sealing faults. The compartments can be considered as three separate accumulations, each possibly with its own oil-water contact. The geochemical interpretations have been used to support drilling farther downdip in the eastern and western compartments on the flank of the structure. An additional 2 million bbl of oil have been found in the eastern compartment and 4.8 million bbl have been found in the western compartment.

AAPG Search and Discovery Article #91003©1990 AAPG Annual Convention, San Francisco, California, June 3-6, 1990