Abstract: Clastic and Stratigraphic Trap Analysis By Computer

V. E. Althaus

The program CLASTIC does comprehensive analysis of clastic sections. It calculates 86 different sand/shale parameters, analyzes the results, presents them graphically, and stores the calculated data for eventual mapping of whatever part of the data is indicated. A search for potential stratigraphic traps is made on the calculated data.

The program matches lithology data to formation boundaries and calculates 86 sand parameters. These calculations consist of sand percent, average sand thickness, sand/shale ratio, shale/sand ratio log₂, weighted-mean position and position deviation. The program then takes 10 imputed increment values, for instance, 0, 5, 10, 15-45. It calculates the number of sands and the net of sands equal to or greater than each of these thicknesses and the number of sands and the net sands between these thicknesses. At the same time it calculates the number of shales and the net shales equal to or greater than each of these thicknesses and the number of shales and the net shales between these thicknesses. This amounts to 20 sand counts, 20 shale counts, 20 sand nets and 20 shale nets, in all, 80 items of data. All of these data are printed out under appropriate headings and at the same time stored on tape with one retrievable set of data for each problem. A graphic presentation is made of the four types of nets showing how the sand net, the shale net, the sand count, and the shale count trend toward zero as the netting increment is increased. The graph also shows the number of wells that still show a net sand or shale. This indicates how much of a map will be covered with contours. The graphic output is analyzed by the user to see which of the computed maps will show the lithology or pinchout features to be mapped and only those data are loaded to the contour program for final mapping. Usually four or five maps out of the computed 86 will suffice.

The program also does stratigraphic trap analysis of the computed data. It selects each well on the map, finds the closest well in each quadrant around that well within a user-specified distance, and then asks the questions: "Is that well higher than this one? Is it thinner? Does it have fewer sands? Does it have less net sand?" If the answer for each question is yes, the area between wells is flagged as a potential spot for a trap and is rated according to how well it compared to defined standards on these four items. The potential-trap locations and ratings are shown on one of the sand maps.

AAPG Search and Discovery Article #90972©1976 AAPG-SEPM Annual Convention and Exhibition, New Orleans, LA