Shear and Compressional Velocities of Overpressured Shales, Louisiana Offshore: Effects of Lithology and Effective Stress

Bruce S. Hart

Wireline and core data show how shear and compressional velocities (V_s and V_p) of overpressured shales from the Eugene Island Block 330 Field (Offshore Gulf of Mexico) are affected by lithology, and effective stress. From 2.1 to 2.5 km depth, V_p ranges from 1750 to 2600 m/s, V_s ranges from 500 to 1100 m/s, and Poisson's ratio is about 0.44. Plotting V_p against V_s for shales shows a strong linear trend that is close to, but different from previously defined trends. For sands, 3 different fields are distinguishable, and these can be related to fluid content, interbedding and shaliness of the sands. Compressional velocities for these deposits are primarily a function of porosity, shaliness (V_sh) and effective stress. For narrow ranges of effective stress, V_p increases with V_sh to about V_sh ^bcong 30 %, then V_p decreases with increasing shaliness. As effective stress increases, V_p increases for all lithologies. The trends seen in the data from these overpressured sediments also fit sediments from the stratigraphically higher, normally compacting (hydrostatically pressured) part of the section in a nearby well. About 3/4 of the overpressure is due to compaction disequilibrium, and the remainder is due to some other mechanism. The reduction in effective stress due to this other cause of overpressure would not have significantly affected V_p. Since there is a good linear relationship between V_p and V_s V_s would not have been significantly affected either. This raises the possibility that the high Poisson's ratio for these shales is an inherited function of depositional processes (undercompaction due to rapid fine-grained sediment accumulation).

AAPG Search and Discover Article #91019©1996 AAPG Convention and Exhibition 19-22 May 1996, San Diego, California