PETROLEUM GEOLOGY OF THE CUSIANA FIELD, ILANOS BASIN FOOTHILLS, COLOMBIA

Cusiana field is located in the Llanos Foothills, 150 mi (240 km) nort heast of Bogota, Colombia. Light oil, gas, and condensate in Cusiana o ccur at drilling depths that average 15,000 ft (4575 m) in an asymmetr ic, hanging-wall anticlinal trap 15 mi (25 km) long and 3-4 mi (5-6 km ) across, formed during the Miocene-Holocene deformation of the Easter n Cordillera. Top and lateral seals are provided by marine mudstones o f the Oligocene Carbonera Group, and support a hydrocarbon column of o ver 1500 ft (460 m), Biomarker data from the hydrocarbons indicate a m arine mudstone source interpreted to be the Turonian-Coniacian Gacheta Formation. Over 50% of the reserves occur in upper Eocene Mirador For mation sandstones, which were deposited predominantly in estuarine env ironments, Additional, deeper reservoirs include estuarine sandstones of the Paleocene Barco Formation and the shallow-marine Santonian-Camp anian Upper Guadalupe Sandstone. Porosity in Cusiana is relatively low Good permeability is retained, however, because the reservoirs are pu re quartz-cemented quartzarenites that lack permeability-reducing auth igenic clays and carbonate cements, Core and well test analyses indica te matrix permeability, not fracture permeability, provides the high d eliverability of Cusiana wells. Cusiana hydrocarbon phases exist in a near-miscible, critical-point state, Reservoir simulation indicates ve ry high liquid hydrocarbon recoveries should be possible from all rese rvoirs by using the reinjection of produced gas to maintain reservoir pressure and to vaporize residual Liquids, The held contains up to 1.5 MSTB of hydrocarbon liquid reserves and 3.4 Tcf* of gas.