Recognition of Compartmentalization and Reserve-Growth Potential in a Structurally and Stratigraphically Complex Reservoir, Barrancas Norte Field, Cuyana Basin, Argentina

Abstract

Abstract Compartmentalization of the Barrancas Norte reservoir (Potrerillos Formation) is a function of genetic stratigraphy and structure and was recognized from integration of this geology with fluid-flow trends. Lacustrine shale flooding surfaces that lie over thin sandstone-rich channel and delta-front sandstones produced vertical fluid-flow barriers. Lateral flow barriers are caused by depositional facies changes, normal faulting, and erosional unconformity onto a basement high. Barriers are recognized from interpretation of multiple oil-water contacts and structural position of fluids, and aquifer influx variability between genetic units. Water over oil at original conditions was a first-order tool followed by fluid movement delineated in the aquifer encroachment model. Modeling of the reservoir resulted in the determination of four reserve-growth concepts, including updip attic oil, incompletely drained compartments, off-structure step-out, and facies and fault-controlled untapped compartments. Additional criteria were also applied to assess the viability of new well locations. First, the target net pay was maximized by locating wells in areas where many channel axes might be overlain spatially. Second, wells were located such that they would target multiple stratigraphic intervals, thus reducing the risk involved in drilling a well. Third, wells were located in conjunction with the aquifer influx model so that the wells targeted only unswept intervals. Introduction The Barrancas Norte field is located in the Cuyana Basin, Argentina. Initial production of the Barrancas Norte field began in mid-1968; however, field development began in earnest in late 1994, when the majority of the wells were drilled. During the initial field life, water production increased from near 0 to more than 50 percent water cut in less than a year. It then fluctuated between 50 and 80 percent until the drilling of new wells decreased water cut dramatically to 20 percent. Since that time, it has again risen to 50 percent and flattened out. The field production indicates that aquifer encroachment into the reservoir is substantial and can occur quickly. This study was undertaken to determine the detailed reservoir characteristics and to assess the possibility of additional field development. Stratigraphy The Barrancas Norte field is located in the Cuyana Basin, Argentina, producing from the Potrerillos Formation. The productive section was subdivided into six genetic units (Fig. 1) that become more sand poor higher in the stratigraphic section. The top of each genetic unit consists of shale with one or more shales within a genetic unit. The lower 50 and 60 genetic units were deposited in a bed-load fluvial system characterized by thick, high net to gross, blocky sandstones. The middle 40 and 30 genetic units were deposited as anastomosing fluvial systems confined by floodplain facies. The upper portion of the section, genetic units 20 and 10, were deposited as fluvial-dominated lacustrine deltas containing narrow digitate channel facies The resulting stratigraphy generates a reservoir with relatively high permeability channel facies generally in the northeast to southwest direction segregated vertically by shales, some of which extend over the entire hydrocarbon-bearing reservoir area. Structural character The Barrancas field lies on the eastern side of a northwest-southeast-trending graben. Within the field numerous faults trend northwest to southeast and have throws of as much as 30 ft. P. 567^