Print this pageC. E. Cruz1, L. M. Albarino1, C. A. Sylwan2, and H. J. Villar3
1Pluspetrol E&P, Buenos Aires, Argentina
2Pan American Energy LLC, Buenos Aires, Argentina
3FCEN (Buenos Aires University) - CONICET, Argentina
ABSTRACT: Source Rocks and Hydrocarbons South of The Santa Cruz Elbow, Bolivia and Northwestern Argentina
The development of a new stratigraphic framework in the Tarija Basin allowed the review of the hydrocarbon source potential within the typically recognized Devonian sequence sets. Nearly 1000 rock samples data (TOC, Rock-Eval pyrolysis and kerogen microscopy) were evaluated for source rock typing and maturity assessment. The samples were collected from several wells and outcrops from a broad area comprising the thrust belt, Chaco Plains and Izozog High, between the Santa Cruz Elbow (Latitude 17°15' S) and the Ramos gas field in Northwestern Argentina (Latitude 22°45' S).
Poor to moderate, and occasionally good source rock qualities have been recorded in most of the evaluated samples, consistent with the findings of Moretti et al. (1995). Typical TOC contents are around 1% or lower, although peak values attain 2%. Variations from thermal immaturity to overmaturity are dependent on stratigraphic position and geologic setting. Kerogen, although predominantly unstructured, shows as type II/III to type III by pyrolysis Rock-Eval, with primary capacity for mixed (gas/oil) generation. Due to the limited organic contents and partly refractory character of the kerogen, effective expulsion of hydrocarbons is believed to begin at relatively high levels of thermal stress (VRE>0.9-1.0%), with prevalence of gassy hydrocarbons. The fair/moderate-TOC-bearing Givetian and Eifelian sections (Iquiri and Los Monos) contain the richest hydrocarbon-prone organic facies in most of the evaluated areas. However, the Givetian beds are disfavored as effective generating sources due to the overall low thermal maturity. The Emsian section (Los Monos-Huamampampa) has poor organic content in Bolivia but is an essentially gas source in Northwestern Argentina (Disalvo and Villar, 1999). The Lochkovian section (Icla-Santa Rosa) recorded the highest organic contents in the Santa Cruz influence area and Izozog High and is probably an active source rock in that zone. Scattered information on the Silurian in the thrust belt indicates low residual organic richness with an advanced overmaturity, but it could have had higher TOC values previously.
Modeling of hydrocarbon generation of the Devonian source rocks (see also Dunn et al., 1995; Moretti et al., 1996) demonstrates that the main episodes of expulsion and charge occurred coupled to the Andean tectonics. A generally low thermal heat flow linked to the Tertiary foreland deposits constrained the maturation process. In the Santa Cruz influence area the Tertiary thickness (less than 2500 m) is not enough to induce maturation of the Eifelian (Los Monos) section, but the Lochkovian interval reaches the maturity required for effective hydrocarbon expulsion (0.9-1.1% Ro). Southwards, in the Río Pilcomayo-Villamontes influence area, an increase of the thermal heat flow is recorded and consequently the Eifelian section becomes an effective source rock. Towards northern Argentina, the Tertiary column is thicker and the thermal heat flow is probably higher. In the thrust belt, the mild thermal regime is counterbalanced by thick orogenic columns up to 6000 m. The Eifelian beds are proven source rocks for oil and gas, while the Givetian (Iquiri) section could be an active source if adequately buried. Overburden during Carboniferous times is viewed to be significant in terms of source rock maturation for the Silurian deposits.
AAPG Search and Discovery Article #90906©2001 AAPG Annual Convention, Denver, Colorado