Regional Rock Physics Evaluation of Oligocene and Miocene Strata, Eastern Llanos Basin (Colombia): Implications for Exploration

Gutierrez, Mario A.
[email protected]

This study presents the rock physics characterization of Oligocene and Miocene siliciclastic successions in the foreland sub-Andean Llanos Basin. One of the important challenges facing geoscientists in the eastern Llanos basin is to validate the economic potential of Oligocene-Miocene rocks using seismic amplitudes. This validation requires the implementation of practical interpretation techniques for calibrating rock properties to seismic data. The predictive use of the seismic data is a direct function of the formation impedance contrast and the lateral variability of the reservoir and associated bounding rocks, and obviously the quality of the seismic data. Using a regional well log database, predictive depth trends are generated to examine whether the expected variation of elastic rock properties would produce an obvious effect on perfect seismic data. Later, forward modelling is applied to investigate whether the seismic field data can predict the key rock property changes. The systematic procedure consists of the following tasks: (1) log and seismic quality control and conditioning, (2) time-depth calibrations, (3) analysis of check-shot velocity surveys and calibration of well & seismic processing velocities, (4) generation of synthetic seismograms for key wells, (5) rock typing & upscaling, (6) rock property trend analysis, rock-physics diagnostic, and model formulation, (7) scenario-based AVO modeling to determine seismic response of the postulated reservoir/seal pairs, and (8) use of predictive model to prospect location. The elastic properties of the Oligocene and Miocene rocks in the eastern Llanos Basin are controlled by variations in lithology and porosity. For clean uncemented sands (C1, C3, C5, C7) with large enough porosities (>20%), hydrocarbon effect modeling suggests that reservoir water and hydrocarbons can not be distinguished using seismic properties, due to the relatively low compressibility of the oil (API<25 and very low GOR). In the eastern Llanos basin, acoustic impedance (AI) and Vp-Vs ratio are good porosity and lithology discriminators. Specifically, high AI and low Vp-Vs correspond to silty mudrocks (Guayabo, C4-C8) and low-porosity sandstones, while very low AI and very high Vp-Vs indicate clay-rich mudrocks (Leon, C2, and C8 West). Intermediate AI and low Vp-Vs are associated with high-porosity sandstones (C1, C3, and C5). Over the depth-range expected for the Carbonera and Leon Formations, weak AVO effects are expected.

AAPG Search and Discovery Article #90166©2013 AAPG International Conference & Exhibition, Cartagena, Colombia, 8-11 September 2013