Print this pageTransgressive Deposits of the Hollin Formation, Oriente Basin, Ecuador
JORDAN, DOUGLAS, H. WILLIAM WADE, DOUGLAS SCHULTZ, CHARLES VAVRA, and HUGO REINOSO
The Cretaceous Hollin Formation in the Villano Field (Oriente Basin of Ecuador) produces from transgressive deposits within an anticlinal flap and has an estimated OIP of over 650 MMB. The Main Hollin is the primary reservoir, has a gross sandstone thickness of nearly 500 feet, and consists of fine to coarse-grained, crossbedded quartzarenite to subarkosic sandstones having excellent quality (porosity = 20%, permeability > 1 darcy, Sw=5%) resulting from an abundance of intergranular pores and lack of clays.
The lower part of the Main Hollin contains braided streams deposited during the initial transgression following incisement of valleys into the Chapiza Formation during a lowstand of sea level. These excellent reservoir-quality rocks have been water-bearing in wells drilled to date in the Villano area. The upper part of the Main Hollin, which contains the majority of the oil, was deposited in point bars of a sandy meandering river having individual estimated non-eroded thicknesses of 50 to 65 feet Poorer reservoir quality sandstones and siltstones/shales interpreted as crevasse splay, levee and chute-fill deposits are more common in the upper part of the Main Hollin, The uppermost portion of the Main Hollin is maid by estuarine/tidal deposits having fair reservoir qualities. The lower contact of the Upper Hollin is marked by the first laterally extensive shale (restricted bay), and the upper contact of the Upper Hollin records a marine erosional event (ravinement sure) containing a basal lag. A limestone and shale sequence of the Napo Formation (condensed section) forms the overlying seal. The Hollin Formation compares well with the Pleistocene/Holocene sequence of the Mississippi River in the United States.