Author: Fernando Alegria, Shell Bolivia Corporation
By analysing the hydrocarbon column height plots of the different gas/oil fields it is possible two distinguish two cluster of fields: the first one is related to small gas/oil columns which comprises reservoirs ranging in ages from Upper Silurian to Miocene in relatively shallow depths. The second one involves reservoirs which exhibit world class hydrocarbon column heights, being the Lower Devonian Huamampampa reservoir the best example in fields such as San Alberto, Sabalo, Margarita and Incahuasi, where the Los Monos mudstones controls gas columns greater than 1000m.
On the other hand, there are structures where the Huamampampa reservoir-Los Monos seal pair did not work properly. Therefore, the wells that evaluated those structures found not commercials hydrocarbon accumulations. Huayco and Iniguazu are examples of this failed traps, which are considered as blown traps.
To explain these presence or absence of hydrocarbon accumulations, we have analysed the depth versus formation pressure plots with the objective of determine the role of the reservoir-seal pressure regimens in the drilled structural traps within the Tarija Basin-Bolivia.
We interpret that the small gas columns are held by capillary seal, which exhibit normal pore pressure gradient, while the big gas columns are sustained by hydrodynamic seals, such as the overpressured Los Monos Formation in the multi TCF gas fields from the southern Subandean fold and thrust belt.
Fernando Alegria, Shell Bolivia Corporation
Fernando earned a BSc in Geology from the Universidad Mayor de San Andres, La Paz, Bolivia.
His 30 years of experience in the industry include 7 years at YPFB, 13 years at Empresa Petrolera Chaco, 6 years in BG and 4 years in Shell, where he currently is Principal Geologist at Santa Cruz, Bolivia, responsible for prospect maturation and exploration growth.
He is a member of the Bolivian Geologist Association and Santa Cruz Geologist Association.