Abstract
This research has been carried out in an area of 678 km2 to the southwest of Maracaibo Lake Basin, where the lower La Grita Member of Early Cretaceous Capacho Formation has an average depth of 7600 ft (2.31 km). La Grita Member consists mainly of calcareous dark grey and laminated shale with oil traces, deposited at about 80 ft (24.38 m) which span from uppermost Albian to Cenomanian. This member represents a transgressive cycle associated with the low section of La Luna Formation (main oil and gas source of the western Venezuela). Besides, this research area has some relative flat in the Cretaceous section with little tectonic affectations, which allow developing unconventional reservoirs.
The methodology proposed by Kuuskraa et al. (2013) to assessment an unconventional reservoir play was used in multidisciplinary studies. A total of 35 wells, one well core, several 2D seismic data, electric logs and geochemical analysis were considered to carry out this study. To make petrophysical analysis the methodology of Passey et al. (1990) was used.
The geochemical data in core samples indicated that this interval shows good conditions for oil generation, where the organic matter percent is between 4.5 – 7.5% of TOC, with a maturity profile in oil window (0.5 – 0.9% Ro). The sedimentation environment suggests that the organic matter is marine. Lithologic description of drill core exhibits calcareous shales with muddy appearance. The Member is composed mineralogically by quartz/feldspar (32), calcite (50%), clay (15%) and other minerals (3%). The brittleness index of the member is around 77%. The electric logs in Member exhibit high radioactivity in some zones possibly associated to abundance of organic content. The petrophysical model matches with core samples results, where the intervals observed with high radioactivity were associated with organic richness. The geomechanical characterization in the area reveals that the state of stress in situ was estimated as normal to transcurrent. Mechanical properties of unit show moderate values of Poisson's ratio (0.23 – 0.26), high values of Young's modulus (0.88 – 4.46 * 106 psi) and UCS (6000 – 13500 psi). The Member can be classified as brittle, according to methodology of Grieser and Bray (2007). Finally, the geochemical and geological features of the member allow suggesting a possible oil play in this area. Moreover, the brittleness index and mineralogical analysis show excellent properties to horizontal hydraulic fracking of rocks.
La Grita Member has never been considered as rock generater of oil in the area. However, the member has reservoirs associated where wells produce oil through natural fractures of the unit. In these reservoirs oil production decrease quickly, affected by low permeability of rocks.
