Integrated Interpretation of Amplitude and Density Anomalies of Maracangalha Formation, Recôncavo Basin

Abstract

Nowadays, the major exploratory challenges in Reconcavo Basin are the complex sandstone reservoirs of the Caruacu Member that belong to Maracangalha Formation. The Maracangalha Formation, composed by Caruacu and Pitanga Members, is formed by shales and low porosity sandstone bodies interpreted as complexes of mass flows. In this geological context, the recently drilled well P-8 aimed to test an opportunity where there was a strong correlation between seismic amplitude anomalies and density anomalies derived from gravity data. The low quality of the seismic data in this area combined with the large lithological variability of the Caruacu Member usually implies in amplitude anomaly maps showing disperse forms with reduced or even no geological meaning. However, the analysis of the seismic data allows the establishment of a correlation involving the anomalous seismic amplitudes and the thicknesses of the sandstones. The analysis of the gravity data used to support seismic interpretation has applied several distinct procedures like basement estimate using non-linear inversion, stripping of the main geological unities, interpretation of the residual anomalies, and density estimate through linear inversion. With help from well and seismic data, the resulting anomalies were interpreted as density variations inside the sedimentary section. Thus, negative anomalies were assumed to be related to shale diapirs as well as to troughs filled with less dense sediments. The seismic amplitude and density anomaly maps show great similarity in shape. The most remarkable feature in these maps is a large NE-SW channel located to the southeastern flank of the shale diapir. Such channel was the main target of the P-8 well, whose existence was confirmed by drilling. After the drilling of the P-8 well, it was possible to conclude that, when used as a lithological indicator, the use of an integrated interpretation of seismic amplitude and density anomalies may lead to the geological success of the prospect. Introduction In oil exploration, targets are usually delimitated by seismic interpretation because the seismic method provides higher resolving power than other geophysical methods. However, the complex geology of the mass flows of the Maracangalha Formation reduces the capability of seismic imaging (Fig. 1). Turbitides associated with slides, slumps and debris flows are practically invisible in seismic data. At the studied area only the main structures of the basin can be visualized. Internal variations on the major sequences caused by different lithologies are often not evaluated. The difficulties found in the exploratory development of this section lie in the location of thick sand bodies of the Caruacu Member as well as in their reservoir quality prediction. In classic exploratory models, the location of the larger sandstone thicknesses is conditioned by troughs usually located along the boundaries of shale diapirs, like in Massape and Lamarao Fields, for instance. Some wells in the studied area show the presence of oil in the Caruacu sandstones. Because only 15% of the sandstones combine the petrophysical characteristics necessary to be considered as reservoirs, the thickness of the sandstones is directly related to the chance of finding a commercial accumulation. Nowadays, there is only one producing oil/gas well, the P-7 well, among the 8 wells drilled in the study area (see Fig. 4 for location). The lateral continuity of these sandstones is not evident in the seismic data due to the discontinuity of the reflections in the Caruacu sequence. This problem can be minimized with the use of stratigraphic markers as auxiliary tools for seismic interpretation. However, no regional marker is available for Maracangalha Formation. We avoid this situation by using biostratigraphic units (biozones) to calibrate seismic interpretation. Such procedure has allowed us to locate these sandstones between biozones NRT004.3 / NRT 004.2. The average thickness of the NRT 004.3 Caruacu sandstones is around 35 m. In this paper we propose an approach that combines the high resolution of the seismic interpretation with a series of gravity procedures that includes 3D gravity inversion for basement relief constrained by both the seismic model and the available well log information; gravity stripping to remove the upper units, which are known and well defined by seismic; and 3D gravity inversion for density distribution to identify anomalous bodies. In addition to more reliable results and the consequent reduction of the exploration risk, we believe that the use of this multidisciplinary approach promotes integration between different geophysical tools. Twelfth International Congress of the Brazilian Geophysical Society SBGf2011