AAPG Annual Convention and Exhibition

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Subsalt 3-D Modelling and HC Reservoir Prediction With Scarce 2-D Seismic Datasets: Can We Obtain Reliable Results?


Lack of geological and geophysical data in exploration process is rather a rule than a random situation. Dealing with 2D seismic data is usual for initial stages of E&P, under a complicated topography, where 3D seismic measurements are physically impossible, when 3D is not economically sound, which is usual for mature oil and gas provinces with relatively small undiscovered reserves, or under a press of low oil prices. Problem of 2D data sets in general is associated with higher structure uncertainty in inter-line space. In case of salt dome another problem is salt shape, which is even more challenging to map in 2D, as well as to delineate subsalt traps. In such situation 3D gravity data give an additional information about salt shape and reservoir distribution. To provide effective and meaningful interpretation of gravity data, the following requirements are to be fulfilled: 1. Physical 3D modelling in real densities must be performed for the full geological sequence from top to basement. 2. Structural and / or property inversion should be performed using observed Bouguer or Free Air gravity data. 3. Inversion algorithm should be redefined. Formal mathematical regularization (for example regularization of the academician Tikhonov A. N.) must be substituted by geologically driven one, which implies that prior information does not only constrain but guides the inversion, resulting the unique solution of the inverse problem. 4. Maximum available G&G information like seismic, well log, petrophysical and other data must be involved into the joint inversion. We will present two examples from different salt basins. These examples show application of the described approach for the cases when (1) initial 3D model is built on the base of 3D seismic, well logs and petrophysical data; and (2) for the case when initial 3D model is built using 2D seismic lines and general petrophysical relationships for geological sequence of the area (no log data used). For both cases 3D modelling was performed basing on joint inversion of 3D gravity data with seismic and additional available geological and geophysical information. We'll compare two workflows and will discuss validity of the inverted 3D models, as well as the results of their posterior verification by exploration and production drilling.