NIBBELINK, K. A., M. C. SORGENFREI, and D. E. RICE, Amoco Production Co., Houston, TX

ABSTRACT: Yombo Field Exploration Model, Peoples Republic of the Congo, West Africa

Yombo field in the Congo is sourced from the lacustrine shales of the presalt rift stage and produces from the Albian and Cenomanian, postsalt, Sendji carbonate and Tchala Sandstone. The Yombo prospect exploration model included an upper Sendji stratigraphic trap with two components and a structural nose.

The buried hill component of the trap is formed by topographic relief on the reservoir below the top Sendji unconformity. The lower Sendji slump blocks provide a high on which the upper Sendji grainstone shoal facies develop. Both depositional relief and erosion during the top Sendji unconformity contribute to the topography. An isochron thick in the overlying Tchala valley-fill sediments defined a drainage pattern on the unconformity around the buried hill of the underlying upper Sendji.

The facies change component is formed by the pinch-out of the grainstone shoal reservoir facies into porous, but impermeable lagoonal dolomite interbedded with anhydrite and shale. Capillary pressure measurements on the 16% porosity, 0.1 md permeability lagoonal dolomite, along with pore throat radius and buoyancy calculations, demonstrated this facies could trap a significant column of low-gravity oil at shallow depth.

The Tchala Sandstone contains several separate hydrocarbon accumulations. A stratigraphic trap in the lower Tchala is formed by marine and tidal channel sandstones pinching out into lagoonal shales. The nearshore marine sandstones of the upper Tchala contain additional hydrocarbons in structural and stratigraphic traps. The stratigraphic pinch-outs that cross the Yombo nose trap a significant hydrocarbon accumulation, even though the four-way structural closure is relatively small.

AAPG Search and Discovery Article #90990©1993 AAPG International Conference and Exhibition, The Hague, Netherlands, October 17-20, 1993.