SUBSURFACE HIGH RESOLUTION SEQUENCE STRATIGRAPHY VIA WELL-CUTTINGS

Most academic studies of carbonate platforms have focused on 2-D analysis using outcrop data, while subsurface studies in industry have been at the scale of individual oil fields. Well-cuttings analysis largely pre-dated modern carbonate facies analysis, sequence stratigraphy, reflection seismic and advanced down-hole logging techniques. These more high tech methods have resulted in well-cuttings being neglected as an important data source for subsurface analysis. However binocular analysis of well-cuttings for relatively shallow wells (0-4000 ft.), can provide detailed vertical facies successions when tied into the wireline logs, and can then be used to generate high resolution sequence stratigraphic frameworks for the subsurface, at a higher resolution than is available from other methods. The coarse fraction (1-2 mm) of the cuttings for each sample interval are washed, acid etched, and examined under binocular microscope. For each sample interval the lithofacies present are separated into groups while wet in a petri dish, and counted to determine relative abundance, which is recorded on a data sheet, and computer plotted against depth for each well to form a percent lithology log. Digitized wireline logs are plotted and the cuttings-percent logs and the wireline logs slipped (typically 10 ft or so) for the best match to take into account drilling lag and mixing. Lithologic columns are constructed from a combination of the cuttings percent logs and wireline logs. These are used to construct sequence stratigraphic cross sections through the basin and into the outcrop belt reference sections. These cross-sections show the detailed distribution of the various facies across the basin, commonly with a resolution of 10 feet. In the Mississippian, sequence boundaries in the wells are placed below lowstand sands, red beds, or eolianites and through sample intervals with caliche. Three dimensional, regional subsurface analysis of carbonate platforms using high resolution sequence stratigraphic methods involving well-cuttings promises to be the next step in study of these important geological features.