Abstract The aim of this report is to describe a Middle Devonian dolostone reservoir in western Canada that has produced more than 57 million barrels of oil and water from moldic-pore reservoir facies with low permeability. The Slave Point Formation consists of six carbonate depositional facies, the relative proportions of which change in response to location on the basement paleotopographic surface. The most significant porosity in Slave Point dolostones is moldic porosity that formed by leaching of fossil fragments; not all Slave Point facies contain fossils. The distribution of fossiliferous carbonate facies, and moldic pores, is ultimately controlled by basement paleotopography. The main conclusion is that there is not a good correlation between permeability and porosity in these rocks. Permeable zones are restricted to dolostones that have touching moldic pores—a constraint that has limited oil production to fossiliferous-carbonate belts that fringe Precambrian granite-basement highs. In contrast, expanses between basement knolls accumulated mostly carbonate mudstone. Mudstone lacks fossils and consequently does not contain moldic pores. The simplest description of this reservoir is that it contains pods of permeability that are surrounded by less-permeable rock, a description suited to many carbonate reservoirs. The desired impact of this work is to draw attention to methods for estimating the probability of finding more-permeable or less-permeable facies types at different locations within these types of reservoirs. This approach would lead to design of more efficient exploration and production programs in complex moldic-pore carbonate reservoirs. Both oil and water are produced from these dolostones. Understanding the effects of high fluid flow in high-permeability zones can enhance oil recovery and reduce water production from similar vuggy-dolostone reservoirs.

Abstract Limestone and dolomite reservoir rocks are by far the more important producers of oil and gas in Mexico compared to sandstone reservoirs. Many of the carbonate reservoirs are complex in that they display three types of pores: matrix porosity; vugs, and cavernous porosity; and fracture-induced porosity. This combination of three types of porosity can be described as “triple porosity.” Each pore type presents a different pore-throat size range and each has a different effect on permeability. In order to better characterize these reservoirs, this paper proposes to visualize the relative abundance of each of these three pore types by plotting them on a triple-porosity pore-type ternary diagram ( Fig. 1 ). Figure 1. Triple-porosity pore-type ternary diagram showing comparisons with previous dual porosity methods. Also shown are areas on the diagram where the main facies of carbonate reservoirs in Mexico plot.

Abstract Basement rocks tend to have negligible primary (rock matrix) porosity and, where present, almost all porosity is fracture-related. An insight into fracture type and distribution is therefore of importance when attempting to predict potential flow zones in hydrocarbon reservoirs. In the absence of core, borehole images provide a proven means of characterizing fractures within basement rocks. This paper will concentrate upon the classification and characterization of basement structural elements, including fractures, faults and breccias, versus lithological elements, such as foliations and intrusion boundaries from borehole images. In addition, suggestions of how to differentiate present-day in-situ stress indicators from natural fractures will be made as these features may provide important information regarding open fracture set orientations. Examples of microresistivity and acoustic borehole images acquired in fractured igneous basement rocks from oil wells in Yemen will be presented together with a suggested methodology for their interpretation. The discussion will be based on hydrocarbon reservoirs; however, the methodology outlined can potentially have wider applications in, for example, groundwater pollution prevention schemes or groundwater extraction, radioactive waste disposal, geothermal energy resources and deep-drilling research programmes.