American Institute of Mining, Metallurgical, and Petroleum Engineers, Inc. This paper was prepared for the 43rd Annual Fall Meeting of the Society of Petroleum Engineers of AIME, to be held in Houston, Tex., Sept. 29-Oct. 2, 1968. Permission to copy is restricted to an abstract of not more than 300 words. Illustrations may not be copied. The abstract should contain conspicuous acknowledgment of where and by whom the paper is presented. Publication elsewhere after publication in the JOURNAL paper is presented. Publication elsewhere after publication in the JOURNAL OF PETROLEUM TECHNOLOGY or the SOCIETY OF PETROLEUM ENGINEERS JOURNAL is usually granted upon request to the Editor of the appropriate journal provided agreement to give proper credit is made. provided agreement to give proper credit is made. Discussion of this paper is invited. Three copies of any discussion should be sent to the Society of Petroleum Engineers office. Such discussion may be presented at the above meeting and, with the paper, may be considered for publication in one of the two SPE magazines. Abstract A general equation for penetration rate in shales is formulated, and the constants in the equation evaluated by a regression analysis of penetration rate data from six Offshore Louisiana wells. The correlation assumes that penetration rate is proportional to weight on the bit, rotary speed, and a hydraulics term, each raised to a fixed power. A penetration rate normalized for changes in these variables is found to decrease with increasing differential pressure and with increases in a tooth wear index. The drillability of shales defined as the drilling rate at some standard operating condition decreases with increasing depth but increases when the pore pressure increases because of the reduced compaction.