The purpose of the corehole programme was to confirm the existence of an active hydrocarbon system to justify and support a seismic acquisition programme in first half 2014 to better characterise basin geometry and source and reservoir rock development, with a view to ultimately characterising resource potential and size. Both coreholes achieved this aim by not only identifying attractive source and potential reservoir rocks but also by confirming oil generation with the existence of “free oil” in multiple core samples in holes over 100km apart.
The Cardozo Chico E-1 corehole reached a total depth (TD) of 694m in mid-October 2013. At 457m it intersected a 10m laminated shale which was sampled. Total Organic Carbon content (TOC) averaged 3.32% with a maximum value of 11.6% and amorphous kerogen ranging from 40% to 70%. Amorphous kerogen is more prone to oil generation as evidenced by free oil observed in several samples. Maturation, based on vitrinite reflectance measured 0.76 to 0.78, is also within the peak oil window. Strong streaming fluorescence observed in two of the three samples confirmed oil generation.
The shale is considered to be part of the Permian San Gregorio Formation and correlates favourably with the shale identified in the Canada de Charruacorehole drilled by ANCAP over a 100km to the north, which also discovered traces of oil in similar aged rocks just a few kilometres north of the northern boundary of the Piedra Sola Block.
Cardozo Chico E-1 Permeability 30.9 darcies Cardozo Chico E-1 Permeability 20.5 darcies
The Cerro Padilla E-1 corehole, with a 1,250m planned total depth, reached a depth of 860m when the corehole had to be abandoned due to hole instability issues. Despite repeated attempts to drill ahead it was decided that the best option was to re-drill this corehole with an engineered mud system, on the same pad at the contractors cost.
The Cerro Padilla E-2 corehole spudded in early April 2014 following a detailed operational review to optimise drilling in light of the extra shale source rocks encountered in the Cerro Padilla E-1 corehole, which terminated at 860m due to hole instability issues associated with intersection of the unexpected Mangrullo Shale. Despite encouraging but only limited sampling in this hole, the Mangrullo shale, appears to be an ideal additional mature source rock and represents a major target for the new corehole. However, repeated drilling problems due to technical and logistical issues as opposed to the rock encountered, ultimately led to this hole also being abandoned to focus on the upcoming seismic programme.
In light of very encouraging porosity and permeability results in the Cardozo Chico corehole new sampling and testing of the Achar E-1 corehole drilled some 2 years earlier has confirmed similarly good conventional reservoir potential some 30km to the east.
Testing by Weatherford, measured very high reservoir porosity in excess of 20%, with measured permeabilities up to 2 Darcy and as high as 7 Darcy (2000md-7000md) in a lower sandstone unit.
The Achar E-1 core has also demonstrated evidence of oil seepage in the time since it was initially drilled.
Uruguay Corehole Achar E-1: Oil leaching from Devonian sandstones just below Cordobes Shales (306-315m)
The initial corehole programme achieved its primary objective of confirming the existence of an active hydrocarbon system from more than one ideal source rockproviding very encouraging and what appear to be potentially widespread, very high permeability, conventional reservoir sands.
After the very good porosity and permeability measurements for Cardozo Chico E-1 and Achar E-1 coreholes some 30km apart and the existence of free oil in different formations over much of the Piedra Sola block the requisite pieces of the puzzle are starting to fall into place.