Effective Mapping of Stratigraphic Traps in Sandstones: Depositional Energy Maps and Diagenetic Maps

Daniel E. Shier

Within a sequence of clastic rocks derived from one provenance, virtually all differences between lithologies can be explained by two factors: depositional energy and diagenesis. Either factor may make or break a prospect. Since the two factors typically have unrelated patterns, effective mapping requires them to be mapped independently.

Separation of the two factors with log data is based on the fact that primary shale usually contains a more or less constant proportion of potassium-bearing clays which is accurately reflected on the gamma-ray curve. The most important diagenetic cements, including quartz, calcite, chlorite, and kaolinite, are largely nonradioactive.

Mapping parameters based on the gamma ray reflect depositional energy. Within certain limitations, the SP can be used in the same way. Diagenesis in sandstones can be mapped by eliminating shaly rocks with a gamma-ray cutoff and mapping the porosity patterns in the sandstones. The most effective exploration mapping parameters for either factor do not use an arbitrary cutoff to measure net feet but indicate the overall energy level or porosity of whatever rocks are present.

AAPG Search and Discovery Article #91038©1987 AAPG Annual Convention, Los Angeles, California, June 7-10, 1987.