GEOLOGICAL CONTROLS OF VARIOGRAMS IN A COMPLEX CARBONATE RESERVOIR, EASTERN PROVINCE, SAUDI-ARABIA

To derive meaningful results from a geostatistical study, it is extrem ely important to establish the relationship between geology and variog rams. However, it is not always easy to establish such a relationship, mainly because of the inadequacy of quantitative or qualitative infor mation. Large amounts of reliable well-log porosity data and the detai led geological information from a complex carbonate reservoir located in the Eastern Province of Saudi Arabia provided an ideal case study o f the relationships between geology and variograms. The reservoir unde r consideration is in the form of a gently dipping, elongated anticlin e and consists of three productive zones which have been subdivided in to several lithologically distinct layers. Results, which indicate an excellent match between geology and porosity variograms, are summarize d as follows: (1) use of stratigraphic distances resulted in considera ble improvement in the behavior of variograms, (2) structurally contro lled geometrical anisotropy is the most obvious feature of variograms, and (3) layers consisting of more complex lithologic framework provid ed variograms with relatively greater nugget variances and shorter ran ges. These results provide further insight into geology and form a mea ningful basis for estimation and simulation of the reservoir. They als o could be used as a general guide in the geostatistical study of simi lar carbonate reservoirs.