Distinguishing Between Oil and Gas in Shaly Sands - A Case History

Abstract

Abstract A primary logging technique used for distinguishing between oil and gas in reservoirs is the overlaying of the Compensated Neutron Log (CNL) Compensated Formation Density (FDC) Log. Whereas this technique is very reliable in clean sands, in shaly sands, however, the log must be corrected for the effects of shale. Methods of correcting the CNL and FDC logs for the effects of shale have been devised, but though these methods are very reliable, they are neither suitable for use at the well site nor for use in quickly distinguishing between oil and gas. Amoco Trinidad Oil Company (ATOC) adopted a method for correcting for the effects of shale on the CNL and FDC Logs called V SHALE ANALYSIS. This method is a modification of a more sophisticated analysis and field tests have proven it to be reliable. The method is suitable for use at the well site for the quick differentiation between oil and gas bearing zones. Introduction Many of the reservoirs in the oil fields of Trinidad, West Indies (Fig. 1)occur in sandstone formations which contain varying quantities of shale. In this respect, the reservoirs in the oilfields off the East Coast of Trinidad which are being operated by ATOC are no exceptions. Since commencing operations, ATOC has developed three oil fields, the Teak Field, the Samaan Field, and the Poui Field. Of these, the Teak Field, which is infamous for its sand production problems, was the first to be developed. Generally, the reservoirs in the Teak Field were relatively clean sands and, as a result, overlaying the Compensated Neutron Log (CNL) with the Formation Density Log(FDC) gave very clear and reliable distinction between oil and gas zones. The Samaan Field was discovered after the Teak Field and it differed from the Teak Field in that its sands were generally shalier. The shaly nature of the sands in the Samaan Field had an adverse effect on the technique of overlaying the CNL and FDC Logs to distinguish between oil and gas bearing zones. The need therefore arose for a method of correcting the CNL and FDC Logs for the effects of shale. Methods were available which gave adequate correction for the effects of shale on these logs, but they were unsuitable for the quick determination of hydrocarbon contents at the well site, since they either required tedious hand calculations or utilized a computer. ATOC therefore adopted a method called VSHALE ANALYSIS to correct for the effects of shale on CNL and FDC Logs. V-Shaleanalysis is a modification of a more sophisticated computer method called SARABAND 1. This paper summarizes the procedure of the V-Shale Analysis and also describes the result which ATOC has had with it. p. 95