Top Seal Evaluation of Miocene Deep-Water Reservoirs, Southern Gulf of Mexico

Abstract

Integration of core, 3D seismic, and well data provided by the Mexican National Hydrocarbon Commission (CNH) was used to evaluate the top seals of Miocene deep-water reservoirs within the Veracruz Trough in the Southeastern Gulf of Mexico. Deep-water well discoveries in this region have often been a mix of oil, gas, or water; raising some questions about the quality of both the source and the seal rock. Nonetheless, traces of hydrocarbons in some water-bearing reservoirs suggest that hydrocarbons were effectively generated. Therefore, the lack of economical accumulations of oil and gas in these reservoirs is probably a consequence of seal failure and/or traps that were filled to an unmapped shallow spill point.

XPT pressure data from the Pemex operated Kunah-1 and Yoka-1 wells for the Upper and Lower Miocene, were used to construct ‘Excess Pressure’ plots to evaluate both top and internal seals. Subtle but significant pressure contrasts over small depth intervals suggested the presence of thin but effective seal rocks. To assess the seal strength for each case, mercury injection capillary pressure (MICP) laboratory measurements were performed on cuttings for those particular depths.

In general, results from these measurements were concordant with our interpretations from pressure data, although in some cases the estimated entry capillary pressure for some samples was lower than expected. However, top seal capillary leakage does not necessarily equate to trap failure, as there could be a matched flowage into the base of the hydrocarbon column and leakage from the top of the column, with a commercial volume of hydrocarbons retained. In these cases, and in the presence of strong seals, spill points are the controlling factor of fluid contacts. Using 3D seismic mapping to define structural closures in both Kunah-1 and Yoka-1 wells we have identified the structural spill points for various Miocene reservoirs. There are relatively few faults in the large folded anticlines tested by the Kunah-1 and Yoka-1 wells.

Seal rocks’ effectiveness was ultimately assessed and defined utilizing various conventional seal classifications (Sneider, Dawson and Almon, Sales, etc.), which are primarily based on the results of MICP measurements and/or the control of the spill points. We conclude that Miocene seal rocks are effective for the observed columns of oil and gas in the southern Mexico deep-water areas but some capillary leakage is probably occurring.

AAPG Datapages/Search and Discovery Article #90350 © 2019 AAPG Annual Convention and Exhibition, San Antonio, Texas, May 19-22, 2019