Abstract: Facies Architecture and Production Optimization in the Upper Cretaceous-Upper Eocene, Operational Unit K1, Apiay Field, Llanos Basin, Colombia

Nieto, Jorge E. - Ecopetrol and Noel Tyler - Bureau of Economic Geology

This integrated reservoir study in Apiay field documents the presence of more than 57 MMSTB of unrecovered mobile oil in an unperforated reservoir (K1 operational unit) which contains 87.5 MMSTB of OOIP. Apiay field is located in southwest Llanos Basin in Colombia and its structure consists of an elongate anticline trending N30°E being cut by reverse faults generated by compensation of a right lateral convergent shear that produced the principal structural features in the area (Fig. 1). Apiay Field produces from an upper Cretaceous to upper Eocene fluvial and transitional marine system, although since its discovery in 1981, production has come exclusively from the massive fluvial sandstones of the K2 unit.

The Operational Unit K1 was formed by the deposition of sandstones and shales of deltaic, shorezone and braided origin. As is typical of these types of environmental settings, these individual sand bodies are more laterally and vertically heterogeneous compared to the underlying K2 unit. Varying fluid (oil gravity) and pressure measurements testify to the unit's complexity and discontinuity. A high production decline and the necessity of finding additional reserves from uncharacterized reservoirs provide the justification for developing a multidisciplinary approach to understand and predict facies architecture in the K1 unit and evaluate its potential for additional oil.

The K1 operational unit was divided in six genetic units from detailed stratigraphic correlation and vertical seismic resolution. Facies distribution demonstrates that the lower K1 operational unit was deposited in a fluvial-dominated delta during the Cretaceous transgression. The middle K1 formed in a mesotidal shorezone system which evolved into a fluvial-dominated deltaic system in the middle part. Finally, the upper K1 was discordantly deposited above the upper Cretaceous and records braided fluvial stream sedimentation in the late Eocene. Figure 2 shows the combined interpretation for tile K1M2 unit (Middle K1) where a typical delta geometry with amplitudes ranging between 40 (light) and 400 (dark) units is shown. The amplitude versus petrophysical properties crossplot showed a direct correlation between seismic amplitude and gamma ray response and a reverse correlation with net sand. This suggests that low amplitude zones correspond to the depositional axes in the system and obviously to the areas richest in sand. This information is taken to generate the final facies model (right of Figure 2) integrating log response, petrophysical properties, engineering data and seismic response.

The total volume of original oil in place for the different reservoirs was calculated using the hydrocarbon pore volume (HPV) mapping for each genetic unit. HPV maps are considered to be the input to locate potential untapped reserves and in this study were constructed using the net sand grid for control. This was required by the fact that lithologic facies strongly affect water saturation. Finally, with the location, quantification and description of the different oil compartments within each genetic unit, a detailed program of recompletions, stimulations and infill drilling was planned for the current and future development of the field to improve oil recovery.

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