Whereas it is true that porosity tends to increase as amount of dolomite increases (Figure 6), it generally does so for the following reasons. In contrast to earlier postulates on the subject (e.g., Murray, 1960 Weyl, 1960), the process of dolomitization of a pre-existing limestone does not automatically create secondary porosity. Clay-mineral cements are extremely rare in carbonate reservoir rocks, and therefore, need not be considered here. The best way to determine the extent of pore-throat restriction in the rocks under consideration is by examining the rocks petrographically in thin-section. Calcite cement overgrowths on crinoid fragments can significantly restrict pore throats as well ( Figure 5), which is why many crinoid-rich Mississippian limestones are of low-permeability nature. In limestones, particularly in grainstones, for example, the nature of pore throats and their effect on permeability is controlled by the size of the particles in the rocks, and more importantly, by the distribution of any remaining earlier-precipitated cement in the pores that was not dissolved (Figure 3F). Over-riding such generalizations about the relationships among pore types, permeability, and depositional environments of the limestones is the importance of pore throats in the rocks (Wardlaw, 1976). Only study of cores/cuttings and thin-sections can resolve the possible reasons for such variations between wells. As a corollary, variations in porosity and permeability from well-to-well within a given zone may be a consequence of different depositional environments in that zone and/or from differing extents of porosity generation versus occlusion between wells. On the other hand, if intraparticle pores are the only pore types present, then porosity (and hydrocarbon storage volume) might be high, but permeability would be low (notwithstanding fracturing). Skeletal sands shoals wherein the particles mainly are foraminifera, which are common in Midcontinent Pennsylvanian limestone reservoirs (Wilhite and Mazzullo, 2000), may be highly porous and permeable because of the presence of interparticle pores, and within the forams, of intraparticle pores as well (Figure 3B). Notwithstanding porosity associated with dolomitization (discussed later), limestones deposited in tidal-flat environments commonly contain a specific type of vuggy porosity referred to as fenestral pores (for example, birdseye pores, which is one type of pinpoint porosity: Figure 3C), and unless fractured, such rocks may have decent porosity but limited permeability. In such cases there may be ample hydrocarbon storage volume in the pores in the rocks, but in the absence of fractures, there is little interconnected porosity. On the other hand, however, high porosity but low permeability may characterize a carbonate sand (limestone) reservoir wherein only the particles have been dissolved (for example, in cases where the reservoir contains only molds of oolites referred to as oomoldic porosity or by the older term oocastic porosity: Figure 3D). For example, carbonate sands (lime grainstones), deposited in high-energy environments such as oolite shoals or skeletal sand shoals, commonly have high interparticle porosity (Figure 3A) and attending relatively high permeabilities. In many cases there is coincidence between the types of fabric-selective pores present in the rocks and the depositional environment of the rocks, which serves as an important guide in evaluating permeability and potential recoverable reserves from the reservoir, and in deciding on what stimulation procedures to use but only if one knows the depositional environment of the rocks from study of subsurface samples. Vugs may originate either by wholescale dissolution of parts of the rock or by dissolutional enlargement of fabric-selective pores (Figure 3E).
The size of vugs (Figure 4) varies from small (but larger than component particles in the rocks) to caverns or cavernous porosity.
limestones, common secondary pore types formed as a result of post-depositional dissolution variously include exhumed interparticle, intraparticle, fenestral, shelter, and growth-framework pores, all of which are considered to be fabric-selective pores and also not fabric-selective vugs ( Figure 3E) and dissolution-enlarged fractures.
0 kommentar(er)
