Abstract: Quantitative Migration Analysis and Prediction

Symington, W. A. - Exxon Production Research Co.; J. W. Higgins and P. R. Smith - Exxon Exploration Co.

Quantitative migration analysis helps estimate hydrocarbon trap charge and, in some cases, phase (gas/oil). The analysis depends on understanding the entire hydrocarbon system, including source rock distribution, the volumes and timing of hydrocarbons generated, the vertical and lateral migration pathways from the source, and the distribution of traps along those pathways. Exxon has developed proprietary technology for quantitative migration analysis. The technology stresses a map-based approach, allowing identification of both fill and spill pathways, and migration shadow zones. This poster will illustrate Exxon's approach using two examples.

The technology was applied in a mature exploration environment, the Diana lntra-slope Basin in the western Gulf of Mexico. The Diana Basin is located 160 miles south of Galveston, Texas, in over 4500 feet of water. The hydrocarbon system in the basin consists of Lower Tertiary to Tithonian source rocks actively charging Plio-Pleistocene reservoirs. Discoveries in the basin include the Diana oil and gas field, the Hoover oil field, and the South Diana gas field. Major reservoir targets are the Pleistocene sands (targeted by the Rockefeller Prospect, figure 1), the Upper Pliocene "A50" interval (the reservoir at Diana, figure 1), and the Upper Pliocene "P1:10" sands (the reservoir at Hoover, figure 2).

Quantitative migration analysis was undertaken after drilling at the Rockefeller Prospect discovered only moderate quantities of biogenic gas, invalidating the existing hypothesis of thermogenic hydrocarbon migration. A fault bisecting the prospect with a throw of over 700 feet at depth had been thought to be the migration conduit for thermogenic hydrocarbons from deep, poorly imaged source rocks. At the time, Diana was the only other drilled structure in the basin. Several other prospects, including Hoover, were thought to rely on this fault being a migration pathway.

The analysis identified likely vertical migration conduits along the salt-sediment interface at the periphery of the basin. Faults associated with basin-rimming salt appear conducive to vertical migration due to frequent reactivation as the salt ascended. Lateral pathways at reservoir levels were mapped away from these vertical conduits, both present-day and in the recent past. Example maps showing present-day drainage cells and spill pathways for the "A50" and "P1:10" reservoir horizons are shown in figures 1 and 2. These indicate that the Rockefeller drainage cell has no access to salt-related faults, while both Hoover and South Diana can access these faults. This configuration of pathways explains the disappointing results at Rockefeller, and permitted forecasting later successes at Hoover and South Diana.

This technology was also applied in a less mature exploration environment in the Campos and Santos Basins, offshore Brazil. Early Cretaceous, pre-salt source rocks in this area are thought to have charged late Cretaceous and Tertiary reservoirs. In preparation for a recent acreage round, numerous leads were ranked by their relative migration risk. The analysis included thermal modeling to estimate source rock maturity and the volumes and timing of generated hydrocarbons. Likely vertical migration conduits were estimated as areas with thin or evacuated salt, structuring that would focus hydrocarbon migration beneath the salt, and faults possibly offsetting the salt. The lateral access of leads to possible vertical migration conduits was mapped. The ranking of leads was based on their access to vertical pathways with likely recent hydrocarbon charge. The results helped quantify overall exploration risk in the area.

AAPG Search and Discovery Article #90933©1998 ABGP/AAPG International Conference and Exhibition, Rio de Janeiro, Brazil