Volumes, Types, and Distribution of Clay Minerals in Reservoir Rocks Based on Well Logs

Abstract

The paper was presented at the SPE/DOE Unconventional Gas Recovery Symposium of the Society of Petroleum Engineers held in Pittsburgh, PA, May 16–18, 1982. The material is subject to correction PA, May 16–18, 1982. The material is subject to correction by the author. Permission to copy is restricted to an abstract of not more than 300 words. Write: 6200 N. Central Expwy., Dallas, TX 75206. Introduction Reliable evaluation of hydrocarbon resources encountered in shaly clastic reservoirs of low porosity and low permeability is an important although difficult task. Log-derived estimates of the volume, type and distribution modes of various clay minerals, determination of cation exchange capacity (CEC) and Qv (CEC per unit of total pore volume), and properly selected water saturation calculation models assist formation evaluation. Since shaly clastic reservoir rocks require extensive core sampling for CEC and Qv analysis, which is tedious, time consuming, and expensive, attempts have been made to correlate such CEC and Qv data with one specific or a combination of several well logging measurements. The latter include the spontaneous potential, gamma ray, natural gamma ray spectral data, dielectric constant and acoustic-, density- and/or neutron-derived porosity, etc. Constraints associated with these concepts will be reviewed. Discussed in this paper, is an innovative digital shaly sand evaluation approach (CLASS), which provides information on total and effective reservoir porosity, total and effective fluid distribution based on the Waxman-Smits equation, reservoir productivity, silt volume, and volumes, types and distribution modes of clay minerals present in subsurface formations. Both basic concepts and field case examples will illustrate this method. CLAY MINERALS Clay minerals, used as a rock and particle term, describe an earthy, fine-grained, natural material which develops plasticity when mixed with a small amount of water. Such clay minerals significantly affect important reservoir properties such as porosity, water saturation, and permeability. Clay minerals are composed of small crystalline particles which are classified according to their crystal particles which are classified according to their crystal structure. Important ones of interest to the petroleum engineer and geologist are kaolinite, montmorillonite, illite, chlorite, and mixed-layer minerals. They are essentially layered hydrous aluminum silicates which may contain small amounts of alkalies and alkaline earths and have some substitution of aluminum by other cations, such as magnesium, iron, etc. The most common clay minerals, their composition, matrix density, hydrogen index, CEC, and distribution of potassium, thorium, and uranium based on natural gamma ray spectral information are listed in Table 1.Numerous experimental data show that the CEC value of clays is directly related to their capacity to absorb and hold water. Clays of the montmorillonite (smectite) group have the greatest capacity to absorb water and also the highest CEC values. Kaolinite and chlorite have very low CEC, and their capacity to hold water is also low. Shales can be defined as an earthy, fine-grained, sedimentary rock with a specific laminated character. Based on the analyses of 10,000 shales Yaalon describes the mineral composition of the average shale as follows: clay minerals (predominantly illite), 59%; quartz and chert, 20%; feldspar, 81%; carbonates, 71%; iron oxides, 30%; organic materials, 1%; others, 2%. Generally speaking, illite appears to be the dominant clay mineral in most of the shales investigated. Chlorite mica is frequently present, smectite is a common component in Mesozoic and younger shales, and kaolinite usually occurs in small amounts only. Therefore, a typical shaly clastic reservoir rock and/or a typical shale formation may consist of several components. Hence, no universal shale parameter can be used to characterize a specific type of argillaceous sediment or rock. P. 67

Geologic Aspects of Tight Gas Reservoirs in the Rocky Mountain Region

“…The geologist analyzing tight rocks for clay content should attempt to differentiate clays that are within the pore and capillary system from those that are detrital clasts or altered rock grains and therefore part of the rock structure. 19 Both types of clays will affect log analysis, but the clays within the flow system critically influence reservoir stimulation and gas recovery. Clays in pore throats and pores have adsorbed water, will increase pore surface area, and will reduce formation resistivity, gassaturated PV, and gas flow.…”

“…Measurement of cation-exchange capacity is one method to attempt to differentiate the illite potassium response from the structural material containing potassium. 19 However, this method is probably not very accurate if clays other than illite are present. As noted earlier, the secondary clays in sandstone pores were formed after deposition.…”

“…This phenomenon must be considered when any shaly-sand analysis is used that compares gamma response of shales to the volume and type of clays in adjacent sandstones. 19 …”

Log-Derived Evaluation of Shaly Clastic Reservoirs

“…Both clay volume and percent expandable clay layers are determined.' CLASS Epilog and Spectra log response in a siltstone/shale sequence in Ohio 16. 184…”