Unconventional Resource Potential of Cretaceous Source Rocks in the Onshore Cesar-Rancheria Basin, Northern Colombia

Abstract

The onshore Cesar-Rancheria basin (CRB), located in the northwestern South America, presents a limited potential for conventional hydrocarbons with the discovery of one gas field in the 90's. However, organic-rich intervals of ∼150-300 m and ∼70-250 m have been identified in the La Luna formation and the Cogollo group, respectively. Type II kerogen predominates in both units and TOC ranges between 2 to 5%. La Loma syncline with a NE-SW-trending fold axis is a major structure in the Cesar Sub-Basin located in the southern CRB. In this structure, the La Luna formation occurs at a depth of ∼2-3.3 km and the Cogollo group at a depth of ∼3-4.4 km. The purpose of this study is to evaluate the potential of these high TOC source rocks for unconventional hydrocarbons, considering two major periods of shortening and uplift according to the tectonic evolution of the CRB: 1) an early–middle Eocene, west-to-east shortening event that produced eastward-dipping Cretaceous and Paleocene strata and the erosion of ∼1-2.5 km of the Paleogene and part of the Late Cretaceous section; and 2) a late Miocene–Recent, west-to-east shortening event with major exhumation of the eastern CRB controlled by revers faulting that induced up to ∼3-3.5 km of erosion in the hanging wall of the main fault. We constructed a 3D-basin model in the Cesar Sub-basin using subsurface mapping from seismic and well interpretation, including TOC and hydrogen index (HI) from maps generated by integrating published information and well data. Vitrinite reflectance (Ro) and pressure data were also included to constrain the model. Results of the modeling show initiation of hydrocarbon generation by late Eocene, the area in the axis of the La Loma syncline had major preservation of the Paleogene and Neogene section after the two uplift episodes, and the Cretaceous organic-rich units are within oil and wet-gas window. These undrilled zones correspond to ∼220 km² for the La Luna and ∼430 km² for the Cogollo group. An additional area that was in oil window until the last uplift event correspond to ∼110 km² for the La Luna and ∼216 km² for the Cogollo group. The fluid pressure of ∼5400 psi at 2560 m-deep and ∼ 7800 psi at 3360 m-deep observed in a well close to the axis of the la Loma syncline indicates over-pressured conditions. The described structure, basin history and source rock characteristic would favor the potential for unconventional resources in the southern CRB.

AAPG Datapages/Search and Discovery Article #90259 ©2016 AAPG Annual Convention and Exhibition, Calgary, Alberta, Canada, June 19-22, 2016