Papa

Engineers routinely confront geologic problems in drilling, completion and recovery programs, because geologic factors determine reservoir geometry, size, and internal characteristics. Drilling rates, circulation parameters, hole conditions, and mud characteristics relate directly to internal formation factors. Exploration programs relate directly to reservoir geometry as well as internalreservoir characteristics. A summary of all geologic factors that relate to environments of reservoir deposition facilitates the determination of the types of engineering treatment used to affect oil and gas recovery. Cooperation between geologists and engineers improves the opportunities for maximum reservoir recovery if all variables are considered in any given recovery program. it is thereforebeneficial for operating engineers to be familiar with geologic technology required for maximum oil or gas recovery.
The importance of geology in engineering operations involves relating rock characteristics to reservoir type, geometry, distribution, and quality. Accomplishment of this objective derives from study of rock types by sample and core examination, which establishes the lithology anddepositional environment of the reservoir rock. Specific reservoir parameters including porosity, permeability, and thickness can also be determined by rock study. Information generated by rock studies establishes the size and shape of the reservoir and how its internal characteristics vary within its framework.
Reservoir porosity. permeability and quality studies must necessarily be augmented by welltest information that relates to pressure and production performance. The ability of the reservoir to transmit 1uids, maintain pressure. and produce petroleum under primary, secondary, and tertiary recovery programs is contingent on its geological characteristics and the ability of the geologists and engineers to recognize and exploit these characteristics
Oil and Gas Occurrences
Porous andpermeable sedimentary rocks comprise the bulk of petroleum reservoirs. Sedimentary rocks are the only rock types that generate petroleum, although igneous and metamorphic rocks can serve as reservoirs.
Sedimentary reservoirs consist largely of clastics such as conglomerate and sandstone, or carbonates that make up reefs and banks. They have primary, depositional porosity and permeability and containmost of the world’s petroleum. Fractured or weathered igneous, Metamorphic, or impermeable sedimentary rocks serve as reservoirs much less frequently than depositionally and diagenetically porous and permeable sediments.
Occurrences and production of petroleum indicate that sandstone is more abundant, but carbonate rocks produce more petroleum (Fig. 381). The latter applies because of thesubstantial Middle East and Mexican production from carbonate reservoirs.
For reservoir rocks to be effective in accumulating oil and gas, they must be included in traps. Petroleum source rocks generate oil and gas that migrate into traps where they accumulate. The traps must contain porous and permeable reservoir rocks that are effectively sealed against petroleum escape by surrounding impermeable rocks.Localization of the petroleum is accomplished by the formation of a trap. which can be depositional or stratigraphic. Deformational or structural. or a combination of the two where deposition and deformation combine. Regardless of how favorable a trap may appear. it will not contain petroleum unless it contains porous and permeable reservoir rocks.
Reservoir Rocks
For a rock to be an effectivereservoir, ii must contain adequate porosity and permeability that is properly sealed against erosion or tectonic destruction. Many reservoirs and traps have been generated and eliminated by erosion and deformation in the geologic past. Their occurrences in retrospect are presently manifest as dead oil, petroliferous odor, and light staining in cores and samples, or as dead-oil seeps and asphalt...