Geological Analysis on Three Dimensional Variogram Model

Abstract

AbstractVariogram is a key parameter in geostatistical studies and is the most widely used tool to investigate and model spatial variability of lithofacies, porosity and other petrophysical properties. With the robust variogram calculation, vertical wells and horizontal wells are used to generate the vertical variograms and horizontal variograms respectively. Then a horizontal range ellipse is fitted to determine the spatial range distribution using the ranges from the horizontal variograms. Integrated the vertical range from vertical variograms, a three-dimensional variogram model is established. Followed this 3D variogram modeling workflow, the 3D variogram models in seven sedimentary environments (meandering river delta, braided river delta, meandering river, braided river, inshore, foreshore and tidal flat) are generated using the horizontal and vertical well data. These variogram models estimated on a 0.3 m interval indicate the presence of decimeter-meter scale heterogeneity. Then the regularity knowledge of horizontal and vertical range distributions, major-minor ratio and major-vertical ratio of the reservoirs in these above-mentioned sedimentary environments are summarized. The results show that: (1) for these environments the spherical model is dominant. (2) The vertical variogram ranges are generally no more than 2 m. (3) The horizontal major ranges of sand bar in braided river, distributary channel and point bar in braided river delta, and channel in tidal flat are relatively small, which are generally no more than 30 m. Point bar in meandering river, mouth bar in meandering river delta, sand bar in foreshore and inshore, and sand flat in tidal flat have relative larger horizontal major ranges which are generally larger than 30 m. (4) The horizontal minor ranges of these sedimentary environments is generally less than 20 m. (5) The major-minor ratios of point bar in meandering river, mouth bar in meandering river delta and sand bar in foreshore and inshore are up to 6:1–10:1. The major-minor ratios of other sedimentary environments are no more than 5:1. (6) The major-vertical ratios of point bar in meandering river, distributary channel in braided river delta, mouth bar in meandering river delta, sand bar in foreshore and inshore, and sand flat in tidal flat are generally up to 40:1. The major-vertical ratios of sand bar in braided river and channel in tidal flat are relative small, which are no more than 20:1. Combined with geological data such as outcrops and literatures, the role and significance of variogram ranges in evaluating and measuring the spatial heterogeneity and continuity of geological variables are further analyzed. The results enrich and improve the reservoir modeling knowledge database, and can be used for reference in the geological model grid design, the selections of model parameters especially the range values in the study of geological model of the similar sedimentary environment.KeywordsVariogramRangeHeterogeneityGeological modelStochastic modeling