Variations in Depositional Systems of the Miocene Oficina Formation in the Cerro Negro Area Using a Sequence Stratigraphic Analysis: Application to Siliciclastic Exploration

RODRIGUEZ, ARGENIS, Intevep, S.A., Caracas, Venezuela

The Cerro Negro oil-sands area is located in eastern Venezuela. Although the Oficina Formation is the major oil-producing formation in this region, until now no attempt has been made to analyze and produce a sedimentological model for these sediments using modern sedimentological sequence stratigraphic techniques in which variations in depositional styles could be documented.

Seven continuously cored wells penetrating the Oficina Formation provide the 1955.9 m (6417 ft) of cored section employed in this study. The sedimentological information gathered during analysis of the cored sedimentary sections comes from the study of lithology, trace fossils and bioturbation structures, abundant shell fragments, clay mineralogical assemblages, and microfaunal data. This information provides a basis for recognizing 13 facies as representative of the cored sedimentary intervals and leads one to divide the sedimentary section into four genetically related facies assemblages or stratigraphic units. These facies assemblages, sedimentary structures and textures, faunal assemblages, and clay mineral assemblages are interpreted as being deposited in a deltaic to shallow-mar ne environment. Stratigraphic Unit I represents a deltaic progradational phase. Stratigraphic Unit II represents reworked delta-front sand deposits redeposited as proximal bars. Stratigraphic Unit III represents an interdeltaic bay environment. Stratigraphic Unit IV is a deltaic reoccupational phase. This interpretation is supported by computer-drawn isopach maps of selected sands within each stratigraphic unit, sand percentage, and sand isolith maps of individual stratigraphic units. This deltaic to shallow-marine facies model proposed for Cerro Negro predicts that sand bodies should have different trends, thicknesses,

and distribution, depending upon the stratigraphic unit in which they were formed. Therefore, the model can be used as a guide in exploration studies of this and other areas having similar geological characteristics to those of Cerro Negro.

AAPG Search and Discovery Article #91004 © 1991 AAPG Annual Convention Dallas, Texas, April 7-10, 1991 (2009)