Abstract

The Lusk Strawn field is located in Lea County, New Mexico. Production from Strawn carbonates was established in October, 1960. To date the Strawn interval has produced 19.6 million barrels of oil and 90.7 billion standard cubic feet of gas. Phylloid algal mounds are present in the area of the Lusk Strawn field and they have traditionally been interpreted to represent the reservoir facies. Their presence played a very important role in the development of the various depositional environments surrounding them. The mounds themselves limited the types of other organisms that could live with them. The broken pieces of phylloid algae trapped fine sediment and provided a firm substrate on which nearby organisms could develop.

Ten wells were cored in some part of the Strawn interval in the Lusk Strawn field. Four of these cored wells were studied to produce a detailed reservoir characterization. Macroscopic and microscopic core description and thin section analyses were used to identify grain types, sedimentary structures, pore types and diagenetic features. Based on this information, 6 lithofacies were recognized and grouped into 8 depositional environments.

Cored intervals were tied to gamma ray and porosity logs. Six shallowing upward depositional cycles were identified from the log/core intervals. Each cycle begins with a rapid marine transgression (dark, argillaceous interval) and ends with the shallowest water facies. All cycles are open marine.

In the cored intervals, the porous zones (reservoir intervals) never occurred within the phylloid algal build-ups, but were always in the debris aprons. All porosity was secondary.