Geologically Driven Joint Inversion of Gravity, Seismic and Well Data: A Step Forward in Understanding of Geological Structure and Reducing E&P Risks

Abstract

In spite of being the first geophysical method to be used in oil & gas exploration, gravity is practically not used today for detailed study of subsurface structure and oil and gas prospecting. This is the result of huge advances in seismics, which began with introduction of the CDP method, leading to a chasm between geological outcome of seismic vs. gravity data interpretation. Recent publications evidence that during the last decades we saw considerable advances in gravity instruments and field survey techniques, but not so big advances in the interpretation techniques. Thus filtration, regional-residual separation, analytical upward-downward continuation, Fourier and wavelet transforms are still used for gravity data interpretation in attempt to link directly gravity anomalies with target geological objects or features, which fundamentally cannot be done due to additive character of gravity field and nonuniqueness of geophysical inversion. More sophisticated approaches, which use physical modelling of the subsurface either stop on forward modelling with partial gravity fit to local/regional anomalies, or construct inversion algorithms using A. M. Tikhonov's regularization theory to obtain stable solution. Inappropriateness of the last one is caused by exotic properties of harmonic function as the natural uniqueness class. Instead, the inverse problem should be redefined so the inversion is not only constrained by prior information, but driven by it, so that additional geological information is used as a guiding rule to select the single geologically meaningful model from a space of all possible solutions. Such reformulation of gravity inversion implies fulfillment of the following conditions: full-earth (from top to basement) real density inversion, using observed gravity, quantifying uncertainties for all the geological sequence and involving maximum additional data like structure by seismic, petrophysics, logs, layering according to expected stratigraphy etc. Described approach was implemented in the proprietary Technology and Software of joint inversion of gravity, seismic and well data. Efficiency of the approach is illustrated by case studies for Ukraine, including near-salt exploration in Dniper-Donets Basin, under-salt gas pools delineation in Transcarpathian Trough, study of oil pool in basement of the Northern Flank of Dniper-Donets Basin, and offshore the Black Sea, all of those post-verified by drilling.

AAPG Datapages/Search and Discovery Article #90291 ©2017 AAPG Annual Convention and Exhibition, Houston, Texas, April 2-5, 2017