Primary Migration in Guaricema Field Area, Sergipe-Alagoas Basin, Brazil: ABSTRACT

Abstract

The Guaricema field is situated in a structural low containing a thick deposit of deep-water marine shales. Oil and gas are produced from an isolated lenticular sand body of Paleocene age. In this particular geological situation a local source for the oil can be tentatively assumed. The Guaricema area contains a fair volume of good mature source rocks in close association with the reservoir. The organic carbon content is above 1.0% and the organic extract averages 4,400 ppm. Gas chromatograms and carbon isotopes data characterize the oil as mature and derived predominantly from marine sapropelic organic matter. Excellent correlation between the oil and bitumen extracted from the adjacent shales shows those shales are the source rocks for the oil. The composition of the extracts also provides evidence of primary migration by systematic changes toward the reservoir (chromatographic effect). In particular, the amount of resins and asphaltenes decreases drastically. Close to the reservoir, the organic extracts are similar to the reservoir oil. The total volume of effective source rocks in the Guaricema field area was estimated in 3.9 km3. The shales still contain 50,000,000 m3 of oil. As the original oil in place in that field was 16,000,000 m3, we concluded that the primary migration was very effective, exceeding 20%. The high efficiency of the primary migration was probably favored by the close association between effective source rocks and reservoir. Both lateral and vertical primary migration were effective. Any mechanism to explain the primary migration in the Guaricema area must account for the high efficiency here evidenced, including the ability of causing much more concentration than dispersion of the hydrocarbons. Migration in an oil phase seems to be the more likely mechanism. End_of_Article - Last_Page 570------------