--> Abstract: Exploration for Deep Gas and Oil in the Central Golfo San Jorge Basin, Argentina, by E. E. Strelkov and P. A. Lafourcade; #90933 (1998).

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Abstract: Exploration for Deep Gas and Oil in the Central Golfo San Jorge Basin, Argentina

Strelkov, Esteban E. and Pedro A. Lafourcade - YPF

A deep, overpressured, gas and light oil play is under exploration in the central part of the Golfo San Jorge Basin, in the province of Santa Cruz, Argentina. Seismically calculated depth to the economic basement ranges 8.5 Km and the deepest well drilled reached a T.D. of 5,160m. With the use of technology now available to the industry, deep fractured clastic reservoirs can be explored with a reduction of risk, and fractured zones can be visualized and interpreted with more accuracy. The petroleum system is Pozo D-129 Fm. - Mina El Carmen Fm. (!) and this system is composed of the following elements:

a) Source Rock: The Pozo D-129 Fm is a lacustrine black shale of Lower Cretaceous age, deposited during late rift - early sag stages of the basin. This formation also includes tuffs, thin limestones and, towards the flanks, sandstones. Average TOC is 2.5%, and vitrinite reflectance index (Ro) is 2 at a depth of 4,400m. Organic matter is Type I, amorphous, algal, with a small percentage of Type III.

b) Reservoir Rock: The Mina El Carmen Fm is of Albian age, and is composed of tuffs, tuffaceous claystones and sandstones, deposited within an alluvial - lacustrine setting. Sandstone maps indicate that coarse clastics distribution was controlled by faults and concentrated in the downthrown blocks. Preserved porosity is low (< 15%) unless its increased by fractures and dissolution. The overlying Comodoro Rivadavia and Yacimiento El Trébol formations also have reservoir quality sandstones.

c) Seal Rock: Claystones and other fine grained tuffaceous rocks in the Mina El Carmen, Comodoro Rivadavia and Yacimiento El Trébol formations act as seal rocks for individual reservoir intervals. The fine grained lithologies in the Pozo D-129 Fm and the tuffaceous rocks in the Mina El Carmen Fm are capable of forming regional pressure seals as expressed by the regional velocity inversion surface observed.

d) Overburden: Total overburden thickness ranges from 3000m at the time of maturity to over 4,500m at present time.

e) Migration: The system is vertically drained, and migration to reservoirs took place through high angle , east- west striking, extensional faults.

f) Trap formation and Timing: Extensional and oblique extensional fault related structures, combined with stratigraphically controlled seals are the main trap elements. 2D BasinMod models calculated peak expulsion to have occurred in the Late Cretaceous, thus the migration pathways and traps were in place to intercept the migrating oil. Continued subsidence and deposition of Tertiary sediments with reactivation of existing extensional faults produced further compartmentation. The overpressuring encountered is interpreted to be driven by an oil to gas reaction in the source rock. Reservoir conditions in this very deep exploration play are extreme. Measured temperature at 4,500m may be as high as 200C. As porosity and permeability decrease when depth increases, because of mechanical compaction and cementation, reservoir quality is good only where enhanced porosity has been produced as a result of fracturing and dissolution.

AAPG Search and Discovery Article #90933©1998 ABGP/AAPG International Conference and Exhibition, Rio de Janeiro, Brazil