Therefore, drill cuttings typically represent the first, and sometimes only, geological samples from an exploration well. Furthermore, while it is possible to derive measurements of permeability on drill cuttings, generally more accurate, laboratory based, determinations are possible with drill core, which of course can be linked to a detailed geological description of the drill core and petrophysical logs The industry is heavily reliant on data derived from petrophysical logs via devices and methods of increasing sophistication. However, it is important to remember that the majority of porosity determinations from petrophysical logs are derived via the analysis of the mineral plus matrix density or the fluids within a given rock over an interval of depth. Petrophysical logs cannot directly measure void space! Figure 3 summarizes the relationship between various mineral and pore characteristics and various modes of analysis. Note that consists of isolated, unconnected pores and connected pores of ize. Also note that there is significant Total Porosity Neutron Log Total Porosity Density Log -+Humidity-dried Core-determined oi CLAY SURFACE AYERS INTERLAYERS SMALL LARGE ISOLATED BOUND ILLAR WATER WATER WATER I PORE VOLUM SHALE ABSOLUTE OR TOTAL POROSITY Figure 3. A simplified pore-system schematic that relates mineralogy, pore type, fluid type and state, and various means of determining porosity (after Cone Kersey, 1992) Figure 3 implies that both Neutron and Density log could report a higher percentage for porosity compared to the hydrocarbon pore volume, as could laboratory derived assessments of porosity. This is because when samples are prepared for either mercury, or gas porosimetry, samples are initially cleaned using reagents and/or solvents then subsequently dried, with the aim of removing all hydrocarbons. However, sample preparation may inadvertently reduce, or eliminate, the irreducible water content, which may generate an exaggerated porosity value(Figure 3). In contrast, petrophysical log analysis may also include clay-bound structural water; therefore care and caution must be exercised when evaluating data derived from all modes of analysis None of this is meant to imply that petrophysical log determinations and laboratory assessments of porosity are error prone and to be avoided, quite to contrary! They are widely used within the industry; petrophysical logs are discussed later in a 魔(○)( chapter devoted to petrophysical logging. However, every technique has limitations! In a similar way the visual assessment of porosity takes practice, skill and due diligence. The reporting ACCURACY PRECISION RACY and geologist must instill a high level of confidence by creating data that has high levels of both accuracy and precision; Figure 4 Figure 4. Accuracy and precision conveys the point Pore Type Carbonate rocks Because the porosity within a carbonate rock can be the product of diagenesis and/or the conditions of deposition (lucia, 1995), more pore types have been identified for carbonate than siliciclastic rocks (i.e. sandstone). A number of different types of porosity be ed e and in drill cuttings. However, the classification scheme of Choquet and ray(1970) initially subdivides porosity into three groups, known as Fabric Selective, Not Fabric Selective and Fabric Selective or Not Fabric Selective(Figure 4). Within the Fabric-selective group, the character of the grains or crystals(i.e the fabricof the rock defines pores types. In contrast, the Non-fabric selective porosity cross-cuts the rock fabric, and in Fabric selective or not pores may display a fabric control or not2 Therefore, drill cuttings typically represent the first, and sometimes only, geological samples from an exploration well. Furthermore, while it is possible to derive measurements of permeability on drill cuttings, generally more accurate, laboratory based, determinations are possible with drill core, which of course can be linked to a detailed geological description of the drill core and petrophysical logs. The industry is heavily reliant on data derived from petrophysical logs via devices and methods of increasing sophistication. However, it is important to remember that the majority of porosity determinations from petrophysical logs are derived via the analysis of the mineral plus matrix density or the fluids within a given rock over an interval of depth. Petrophysical logs cannot directly measure void space! Figure 3 summarizes the relationship between various mineral and pore characteristics and various modes of analysis. Note that the absolute or total porosity (Figure 3) consists of isolated, unconnected pores and connected pores of varying size. Also note that there is significant difference between total porosity and the hydrocarbon pore volume. Figure 3 implies that both Neutron and Density log could report a higher percentage for porosity compared to the hydrocarbon pore volume, as could laboratory derived assessments of porosity. This is because when samples are prepared for either mercury, or gas porosimetry, samples are initially cleaned using reagents and/or solvents then subsequently dried, with the aim of removing all hydrocarbons. However, sample preparation may inadvertently reduce, or eliminate, the irreducible water content, which may generate an exaggerated porosity value (Figure 3). In contrast, petrophysical log analysis may also include clay-bound structural water; therefore care and caution must be exercised when evaluating data derived from all modes of analysis. None of this is meant to imply that petrophysical log determinations and laboratory assessments of porosity are error prone and to be avoided, quite to contrary! They are widely used within the industry; petrophysical logs are discussed later in a chapter devoted to petrophysical logging. However, every technique has limitations! In a similar way the visual assessment of porosity takes practice, skill and due diligence. The reporting geologist must instill a high level of confidence by creating data that has high levels of both accuracy and precision; Figure 4 conveys the point! Pore Type Carbonate rocks Because the porosity within a carbonate rock can be the product of diagenesis and/or the conditions of deposition (Lucia, 1995), more pore types have been identified for carbonate than siliciclastic rocks (i.e. sandstone). A number of different types of porosity may be recognized in core and in drill cuttings. However, the classification scheme of Choquett and Pray (1970) initially subdivides porosity into three groups, known as Fabric Selective, Not Fabric Selective and Fabric Selective or Not Fabric Selective (Figure 4). Within the Fabric-selective group, the character of the grains or crystals (i.e. the fabric) of the rock defines pores types. In contrast, the Non-fabric selective porosity cross-cuts the rock fabric, and in Fabric selective or not pores may display a fabric control or not! Figure 3. A simplified pore-system schematic that relates mineralogy, pore type, fluid type and state, and various means of determining porosity (after Cone & Kersey, 1992) Figure 4. Accuracy and precision