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A New Method of Making the Thickness Map of the Shallow Sand Body Constrained by Seismic Attribute

Abstract

Sand body thickness map is one of the most important results of geophysical exploration, especially for offshore oil field, it directly or indirectly determines the scale of platform and the oilfield development plan. The conventional flow is always based on 90°phase shifted data or inversion data, and the general steps are as follows: 1) tracking the top and bottom of the sand body; 2) obtaining the gross thickness by subtracting top depth from bottom depth; 3) obtaining the corrected gross thickness by using the well point thickness; 4) obtaining the net thickness by net-to-gross ratio. The flow exists two problems when thickness laterally varies quickly: 1) under certain conditions, top and bottom surfaces tracked by interactive interpretation are not the real interfaces; 2) in the process of correction, some unreasonable values will be introduced because of the mathematical interpolation algorithm. To solve the above problems, a new method for making the shallow sand body thickness map is proposed. The shallow sand body always has better seismic response; therefore, there is a good correlation between the thickness and seismic attributes. Forward modeling results show that the cumulative thickness of sand body is positively correlated with reflection amplitude and negatively correlated with instantaneous frequency when the thickness of the sand or the interbed sand is less than 1/4 seismic wavelength. Such the relationship between the thickness and seismic attributes can be established by means of linear fitting or artificial neural network. In the step of correction, we adopt Kriging interpolation method: the thickness is consistent with well point thickness in the well control area, and consistent with seismic attributes in the outside well control area. The well control area can be obtained by means of variogram function. In this paper, we also give the steps of calculating the effective thickness: 1) calculating the net thickness using seismic attributes and the well point thickness; 2) getting the gross thickness through the net-to-gross ratio; 3) removing the gross thickness under the oil-water interface; 4) calculating the effective thickness using the remaining gross thickness multiplied by the net-to-gross ratio. This method has achieved good results in practical application. This method considers sedimentary facies by seismic attributes, so the obtained thickness map is more agreed with our cognition.