Abstract
Several types of ancient soil zones are present in the reservoirs of the Hamaca Development Area, Orinoco Heavy Oil Belt of Venezuela. It is important to recognize these paleosoils when drilling production wells and when constructing reservoir simulation models. The major reservoir sands were deposited in fluvial and fluviotidal channel systems in Miocene strata that rest directly on Precambrian basement. A thick weathered regolith is locally present on basement. It is composed mostly of etched quartz and kaolinite, and in places bears the structure of the antecedent crystalline rocks. The intense weathering typical of ancient tropical soil processes has removed most radiogenic materials from these regoliths, so they have a gamma ray response similar to that of a reservoir sand. However, they drill extremely slow, using up valuable rig time, and probably will not contribute significant production. Unfortunately, they cannot be readily distinquished from good reservoir sands by seismic methods. Other types of paleosoils are present higher in the reservoir zone. These soils developed in floodplains and backswamps and are characterized by abundant illuvial kaolinite, precipitated siderite spherules, blocky soil peds, and pale colors. They often occur directly on top of channel and splay complexes, and are sometimes overlain by lignites or liginitic shales. These may also have subdued gamma ray response, making them difficult to recognize with MWD gamma ray tools, but their high irreducible water saturation is reflected in their low resistivity response. These paleosoils can contribute to seals and baffles within reservoir intervals, and can also complicate drilling by introducing slow-drilling, less productive zones within the reservoir. These can be recognized in some, but not all, instances with seismic data, when sufficient well control is also available.
