An Updated Semivariogram Atlas for Carbonate Reservoirs

Abstract

Many stochastic (geostatistical) reservoir property-modeling algorithms use the semivariogram as a primary measure of spatial continuity. Generally, the semivariogram is derived from the available reservoir data, usually well logs and/or core data, though occasionally seismic data is also used. In the absence of sufficient available field data (e.g. limited number of wells), the semivariogram used for reservoir modeling may be derived from analog reservoirs or may be treated as an uncertainty during probabilistic modeling. Based on a survey of published semivariograms derived from reservoirs with abundant well control, an effort has been made to correlate the semivariogram range parameters with depositional environment. For example, the horizontal (XY) range parameter is typically 1500 ± 500 meters with little directional anisotropy for ramp, shelf, and platform carbonates. The vertical semivariogram (Z) range is typically 5-10 meters in these carbonate reservoirs. For carbonate buildups, the semivariogram XY range is about 500 ± 250 meters and the Z range about 5-10 meters. The smaller XY range for the carbonate buildup reservoirs almost certainly reflect the generally closer well spacing in these reservoirs compared to the other types of carbonate reservoirs. Other trends based on the published studies include: (1) Limestone reservoirs have an average XY range of about 1700 meters compared to average XY range of about 1300 meters for dolomite reservoirs; (2) there is a slight negative correlation between the age of the carbonate reservoir and the XY range with reservoirs less than 100 Ma have an XY range of about 2000 meters compared to an XY range of about 1100 meters for those with an age of 250-500 Ma; (3) the average XY range anisotropy is low, about 1.4, suggesting that directional trends are minimal in carbonate reservoirs; and (4) the XY range for individual stratigraphic units within some carbonate reservoirs has significant variability. The majority of carbonate reservoirs included in this survey are located primarily in the Middle East, Permian Basin (USA), western Canada, and central Asia.

AAPG Datapages/Search and Discovery Article #90350 © 2019 AAPG Annual Convention and Exhibition, San Antonio, Texas, May 19-22, 2019