Horizontal drilling in a mature hydrocarbon province such as Texas can increase recovery from reservoirs in which a significant proportion of oil or gas in place remains unrecovered by conventional means. Such drilling has been particularly effective in increasing production where production is from natural fiactures, such as in the Pearsall field of the Austin Chalk trend in South Texas. Production can also be enhanced where the reservoir-quality
zone or the oil column is thin or where gravity-drainage production occurs. Although they are not current targets of horizontal drilling activity, many other kinds of reservoirs contain depositional heterogeneities that interfere with recovery by vertical wells. The use of hckizontal drilling to tap multiple reservoir compartments in these reservoirs has significant potential but has yet to be fully tested. This circular summarizes geological and engineering characteristics of selected formations in Texas that have been or are potential targets for horizontal drilling.