Jhon Munoz-Barrera

Subsurface characterization of structural traps in the outer Nunchia foothills, Colombia.

The foothills have been focus of several researches because (1) in their sediments are recorded the uplift history of the mountain belts, (2) these areas accommodate the regional shortening, and (3) these zones are highly prospective for hydrocarbon exploration. Usually in these areas the structural models are generated based on surface geology, 2D seismic and exploratory wells. After a discovery, the models are updated using 3D seismic, development wells and production history.

The Eastern foothill of the Eastern Cordillera of Colombia is highly explored by its hydrocarbon potential. Based on some transverse zones and changes in structural styles it have been divided in three zones (Bayona et al., 2008; Cortés et al., 2006 and Cortés et al., 2009) Northern, Central and Southern Llanos Foothills (Figures 1 and 2). The Central Llanos Foothill is the most prospective zone with more than 3000MMOB of original reserves tested. Eight oil fields are situated in this zone; in the south: Cusiana, Cupiagua, Pauto Complex (Pauto, Floreña, Volcanera and Dele) and Huron oil fields and in the north the Capachos oil field. Between these two areas a region of 130km has been explored without success. Informally this region is composed by the Nunchia and Zamaricote foothills (figure 1). Two important geological features can be highlighted in this region, first the Eastern Cordillera changes its direction from N30E to N-S and second, the presence of new thrust salient that keeps the N30E direction and is composed by the Piedemonte fault system. The area have been interpreted as thin-skinned (Rochat et al. 2003) to thick-skinned (Teson et al, 2013) with some variations between them (Cediel et al; Munoz 2015). Rochat et al. (2003) divided the Nunchia foothills in two main regions, internal and external structure (Figure 2).

This work used 533.5Km² of an exploratory 3D seismic acquired with sparse design, 511Km of 2D seismic, two exploratory wells and surficial geological information located in the Nunchia outer foothills (Figure 1) to (1) recognize the geometry of the structures, (2) identify the variability of the structures along strike, (3) identify which structures involve the reservoir rocks, and (4) define the structural style.

This research presents the first 3D exploratory structural model of the Nunchía outer Foothills. An integration of surface geology, 2D and 3D seismic and well information in ArcGis and Petrel software allowed the surface analysis, seismic interpretation and well correlation. Additionally, the interpretation was exported to Move software to build the structural model and generate the displacement analysis. This tool allowed the identification of interpretation problems. The model was created in time domain because the study area had only two exploratory well separated 3.4km in the dip direction.

The end-member of the Nunchia Foothills can be classified as active-roof duplex, characterized by a thin-skinned structural style. In this zone, the shortening is accommodated in a series of imbricate to duplex structures with different detachment levels. Therefore, this area can be divided in three zones: 1) basal structures, defined as structures that have the lower detachment in the Gachetá Formation; 2) intermediate structures, with detachment in both C-8 (Lower Carbonera Formation) and C-6 (Middle Carbonera Formation), and; 3) upper to surface structures with detachment level in C-2 (Upper Carbonera Formation). Along strike the Nunchía outer foothill become deeper to the north and the distance between the inner antiformal stack and outer imbricate region widens. The well correlation between the foreland and foothills sequence allowed interpreting that the Carbonera, León and Guayabo formations are syn-deformation deposits, with lateral thickness variation, increasing to the west and to the north.

Only the basal structures involve the reservoir rocks. Two structures were identified. The southern structure, Tangara, is a water bearing structure and the northern structure is untested with remain oil prospectivity.