Study Of Petroleum Geology And Reservoir Performance In Block ST, Ecuador

Block ST, located in the East Oriente basin of Ecuador, has 5 producing heavy oil fields. The Block is situated in the center of the basin and east to the Cononaco-Auca-Sacha main trend. There are three main pay zones from base to top: U sandstones (Middle Cretaceous), Ml sandstones and Basal Tena formation( Upper Cretaceous). The Ml sandstones are further divided into MlA and MlC reservoir and U sandstones are further divided into Upper U and Lower U reservoir.This paper studies petroleum geology and reservoir performance characteristics mainly with decline analysis and water cut behavior of individual reservoirs in the block.The following points are summarized from the study:1.There are three kind of traps in the block: structural traps, structural/stratigraphic combined traps and structural/lithological combined traps. Structural traps are dominant in the block. Because of the stable tectonic basement, the folding and faulting activity has not been very strong, most of the anticlines are small with low closure.2. Major pay zones are MlC and Lower U both of which are deposited in amalgamated channels. The sand bodies of the reservoir are of high continuity, high connectivity and high NTG values, the average porosity of MlC is 25% and that of Lower U is 20%, the average permeability of MlC is 4000 md around and that of Lower U is 2500 md around.3. As the high continuity and connectivity of MlC and Lower U sandstones, strong and active bottom/edge water exist in the reservoir, the oil gravity is about 15°API and the formation oil viscosity is from 40-120cp.4. In general, the initial oil rate is very high but declines very rapidly;while the water cut increases sharply;Producing period with high water cut is very long, which is caused by the stable structure, the depositional facies, goodrock properties, strong bottom/edge water and high formation oil viscosity.5. On the other hand, wells in different production zones, different fields and different structural position have different behaviors. Four types of water cut vs cumulative oil curve have been summarized.