| SEISMIC
OVERVIEW
ECOPETROL contracted Western
Geophysical to acquire and process 125 km2 (78 mi2)
of  3-D seismic data covering the Apiay-Ariari area. The data were jointly
interpreted by an interdisciplinary team of ECOPETROL and Western Geophysical
personnel.
The overall topography of
this region is gently rolling farmlands. The acquisition of data was slightly
hampered by pre-existing production facilities around the Apiay and Suria
fields. Table 1 lists a summary of the main parameters used for the seismic
acquisition (Figure 3).
Standard 3-D seismic processing
was done in Bogotá. In addition, random noise attenuation, instrument
phase correction, and time variant spectral whitening were performed in
Houston to achieve a dataset suitable for stratigraphic interpretation.
STRATIGRAPHIC FRAMEWORK
The stratigraphy of the Llanos
Basin, Colombia, is typically described with standard lithostratigraphic
nomenclature (Reyes, 1992). Nomenclature used in this study is consistent
with the operational units of the Llanos Basin as defined by ECOPETROL.
These units are bounded by unconformities and/or lithology changes (Figure
4).
PALEOZOIC-MESOZOIC STRATA
Paleozoic strata lie unconformably
upon Precambrian rocks. The lower units consist of Cambro-Ordovician strata
overlying phyllites. The seismic reflection for this boundary is of poor
quality. The Mesozoic section in the Apiay area consists of Upper Cretaceous
strata, which are Cenomanian-Maastrichtian in age (Castro-Arévalo
et al., 1991). ECOPETROL recognizes two operational units in this formation.
The older K-2 reservoir is the most economically significant. Core analysis
of this unit indicated quartzose, medium- to coarse-grained sandstones,
varying locally to conglomeratic. This unit has been interpreted as braided
stream channel deposits. Overlying the K-2 is the K-1 unit, which consists
of shales with interebedded fine- to |
