Ahr W1 H. Geologyof Carbonate Reservoirs

Ahr W1 H. Geologyof Carbonate Reservoirs

(Parte 4 de 8)

Major importance in reservoir properties if present

Visual estimation of porosity and permeability

Semiquantitative estimates may be relatively easy

Semiquantitative estimates may be easy or impossible; instrumental measurements commonly required

Adequacy of core analysis for reservoir evaluation

Core plugs of 1 - inch diameter may be adequate to determine matrix porosity

Core plugs commonly inadequate; may require whole core analyses (∼ 4 - inch by 1 - foot segments) for large pore sizes

Porosity – permeability relationships

Relatively consistent; may be dependent on particle texture

Highly varied; may be independent of particle texture

Reliability of log characteristics as indicators of depositional facies (electrofacies mapping)

Standard practice that may provide reliable proxies for depositional facies

Not reliable because logs cannot generally detect differences in carbonate grain types or textures

Source : Adapted from Choquette and Pray (1970)

UNIQUE ATTRIBUTES OF CARBONATES11

12 INTRODUCTION

geology is by F. K. North (1985) , Petroleum Geology

A good general reference on carbonate sedimentology is by M. E. Tucker and V. P. Wright (1990) , Carbonate Sedimentology ; and an encyclopedic review of petroleum

1.1What is the difference between a reservoir and a trap?
1.2What units of measure would you use to describe traps? To describe

REVIEW QUESTIONS reservoirs?

1.3What is the difference between a trap and a seal?
1.4What measurements are used to determine the “ effi ciency, ” or capacity, of a

seal?

1.5What differentiates reservoir characterization from reservoir description?

From reservoir engineering?

1.6What are at least three differences between carbonate and terrigenous sand-

stone reservoirs?

1.7What type of data would explorationists use to identify subsurface structural

anomalies in untested areas?

1.8What, according to Focke and Munn (1987) , is a characteristic of carbonate

reservoir porosity that must be taken into consideration when making fl uid saturation calculations with the Archie equation?

1.9Which wireline logs would you use to create depositional facies maps in car-

bonate reservoirs?

1.10What is the main difference between carbonate reservoirs and carbonate

aquifers?

Geology of Carbonate Reservoirs: The Identifi cation, Description, and Characterization of Hydrocarbon Reservoirs in Carbonate Rocks By Wayne M. Ahr Copyright © 2008 John Wiley & Sons, Inc.

2.1 DEFINITIONS

This chapter focuses on the defi nitions of fundamental rock properties, how the properties are used to classify both rocks and porosity, and how fundamental rock properties are related to reservoir properties. It is traditional in geology to use purely descriptive terms for rock properties because objective descriptions are less likely to contain subjective interpretations or biases. This philosophy has merit in most cases, but in the end, the task of the reservoir geologist is to formulate interpretive models for use in exploration and development. In carbonate rocks, reservoir porosity and permeability can be formed by a variety of processes. These processes create the rock properties we describe with rigidly objective terms. Some of the formative processes may have affected reservoir rocks more than once; therefore an accurate reservoir description should incorporate terminology that classifi es the altered properties, the processes that created them, and at least an estimate of the number of times the rock properties underwent change. To produce such a classifi cation requires the use of genetic terms along with descriptive ones. For example, subjective interpretations are required to determine which processes caused diagenetic changes at which times during the burial history of the rocks. Diagenetic porosity can be classifi ed in a totally objective manner, but without interpretations of how, when, and where different diagenetic events changed preexisting pore characteristics it is hardly possible to predict the spatial distribution of the ultimate porosity.

Traditional geological literature includes the terms primary and secondary to describe rock properties. This is not helpful because those terms are ambiguous. Primary may be used in a temporal sense to indicate the depositional origin of rock properties such as grain size, grain composition, or skeletal morphology in the case

14CARBONATE RESERVOIR ROCK PROPERTIES of calcifi ed organisms. Secondary properties traditionally refer to features produced by diagenesis. This oversimplifi ed language creates more problems than it solves because diagenesis is a continuous process that may affect a reservoir rock many times in its burial history. If diagenetic and fracture porosity are “ secondary, ” then how do we distinguish between multiple episodes of diagenesis or fracturing that had major impacts on reservoir characteristics? How do we develop accurate reservoir models that take into account the different times and modes of change? Some texts imply that primary rock properties such as texture and fabric are depositional only. However, texture and fabric can also be used to describe diagenetic properties of carbonate reservoir rocks. Because diagenetic characteristics represent multicycle changes, not just one - time, secondary changes, it can be confusing to classify rock and reservoir properties simply as primary and secondary. It is important to distinguish between timing and mode of origin of the various diagenetic events to determine the history of porosity development. Similarly, it is necessary to distinguish between timing and mode of origin of different fracture sets in fractured reservoirs. The problem of dealing with descriptive terms is made less troublesome if carbonate rock properties and pore categories are classifi ed genetically as products of depositional, diagenetic, and fracture - related processes. Those are end - member processes. Further subdivision of the classifi cations can be made to identify detrital, chemical, and biogenic deposition. Diagenetic attributes can be linked to time and mechanism of change. Mechanically produced fractures, cataclastic textures, mylonites (gouge), ductile folding, and plastic deformation can be related to different times, stress conditions, and material properties. Hybrid properties such as those produced by depositional facies - selective (texture plus - or - minus fabric) diagenesis, or strain recrystallization, or stylolitization, can also be described in terms of time and mode of origin. Time and mode of origin of depositional, diagenetic, and fracture rock properties are, as we will demonstrate throughout this book, critical to understanding the architecture of carbonate reservoirs.

Depositional, diagenetic, and tectonic rock properties, although they are genetic, still represent basic descriptive characteristics of carbonate reservoirs. They can even be thought of asfundamental properties in the sense that other reservoir properties are dependent on them. Properties such as porosity, permeability, and bulk density are dependent or derived properties. Yet another set of properties is encountered in the study of carbonate reservoirs: third order ortertiary properties . Tertiary properties include electrical resistivity and conductivity, acoustic transmissivity, natural radioactivity, and the various attributes measured by most wireline logs, gravity meters, magnetometers, and the seismograph. Those characteristics depend on porosity, fl uid content, radioactive element content, rock density, magnetic susceptibility, and acoustic characteristics, none of which are measures of fundamental rock properties. In describing rock properties, the words primary, secondary, and tertiary have no time signifi cance and they may not be related to deposition, diagenesis, or mechanical fracturing. It is preferable to describe rock properties as fundamental, dependent, and tertiary.

2.2FUNDAMENTAL ROCK PROPERTIES

Fundamental properties of carbonate rocks include texture, fabric, grain type, mineralogical composition, and sedimentary structures. Note that texture and fabric are

FUNDAMENTAL ROCK PROPERTIES 15 not interchangeable terms. Texture is defi ned as the size, shape, and arrangement of the grains in a sedimentary rock (Pettijohn, 1975 ). Among carbonate sedimentologists, texture is sometimes thought of in the context of depositional texture , which forms the basis for several carbonate rock classifi cation systems. Fabric refers to the spatial arrangement and orientation of the grains in sedimentary rocks. It can also refer to the array geometry or mosaic pattern of crystals in crystalline carbonates and the growth form (macroscale) and skeletal microstructure (microscale) of reef organisms. Mineralogical composition refers to original mineralogy. Original mineralogical composition has great signifi cance in the study of carbonate diagenesis and it provides important clues about the chemical evolution of the earth. It is not, however, a reliable clue to the origin and distribution of reservoir fl ow units because carbonates in a wide variety of depositional settings may consist of calcite, aragonite, or dolomite, individually or in mixtures. It is more practical for the reservoir geoscientist to substitute constituent grain type, such as skeletal grains, peloids, clasts, or ooids, among others, for composition. Sedimentary structures are preserved bedforms created by fl uid processes acting on the sediment interface, by desiccation, slope failure, thixotropy, compaction, fl uid expulsion, and bioturbation by burrowing and boring organisms. These defi nitive rock properties are discussed in more detail in the following sections.

2.2.1 Texture

There are many textural terms in the literature on sedimentary rocks, but most geologists today describe grain sizes according to the Wentworth (1922) scale in millimeters, or in “ phi units, ” which are logarithmic transformations to the base 2 of the size (in millimeters). It is rarely possible to disaggregate lithifi ed limestones into component grains; consequently, direct size measurements by sieve, pipette, or hydrometer are limited to unconsolidated sediments. Estimates of grain size can be made from thin sections of lithifi ed carbonates, although the method requires statistical manipulation of grain size measurements to compensate for the fact that two - dimensional microscope measurements do not provide the true three - dimensional grain size. Tucker (1988) and Tucker and Wright (1990) discuss the problem of determining grain sizes from thin section measurements in more detail.

The Wentworth scale (Figure 2.1 ) classifi es all grains with average diameters greater than 2 m as gravel , those with average diameters between 2 m

and 116 m (62μ m) as sand , and those fi ner than 62 μ m as mud . In this context, sand

denotes texture rather than composition. Other terms for gravel, sand, and mud

that exhibits a mosaic of calcite crystals 1 – 4μ m in diameter became known as

include the Greek derivatives psephite, psammite, and pelite, but they are rarely used in modern literature. The Latin terms rudite, arenite, and lutite appear in the comprehensive but unwieldy sedimentary rock classifi cation scheme of Grabau (1960) . The terms appear in modern literature as calcirudite , calcarenite , and calcilutite , indicating carbonate gravel, sand, and mud, respectively. Embry and Klovan (1971) blended rudite with Dunham ’ s (1962) carbonate rock classifi cation terminology to create rudstone in their classifi cation of reef carbonates. Lithifi ed lime mud micrite , a contraction of micr ocrystalline and calc ite , coined by Folk (1959) . Some workers now classify all carbonate mud, regardless of its size and mineralogical composition, as micrite, even though that is inconsistent with the original defi nition.

Much of this “ micrite ” is actually calcisiltite , or silt - sized (62μ m to 3.90 μ m) sedi-
to the sea fl oor as disk - shaped particles 2 – 20μ m in diameter (Milliman, 1974 ). Elec-
but they are not proper chalks

16 CARBONATE RESERVOIR ROCK PROPERTIES ment. Note that chalk is a special rock type that is not generally classifi ed as micrite or mud. True chalk consists of cocolith skeletal fragments, usually in a grain - supported fabric. Coccolithophorids are fl agellated yellow - green algae that produce a spheroidal mass of platelets that become disarticulated after death and rain down tron micrographs of chalk show grain - supported depositional textures without a matrix of aragonite or calcite crystals fi ner than the cocoliths; therefore chalk is not strictly a mud or micrite in the sense of the detrital micrites described earlier. Of course, there are “ gray ” areas. Calcisiltites (lime muds) may contain some cocoliths,

Grain size is not generally as useful for interpreting ancient hydrologic regimes in carbonate depositional environments as it is with terrigenous sandstones nor is grain size consistently related to carbonate reservoir porosity or permeability. Carbonates consist mainly of biogenic particles that owe their size and shape to skeletal growth rather than to a history of mechanical transport, deposition, and arrangement. Most carbonate grains originate in the marine environment where waves and currents fragment, winnow, and sort sediment, primarily along strand plains and on slope changes (usually associated with bathymetric highs) that occur above

Figure 2.1The Wentworth grain size classifi cation. Note that all particles fi ner than sand

Size in mmParticle NameAggregate Name

Boulder

Cobble

Pebble Granule

Gravel

SandSand

Mud Silt

Clay

4 m 2 m

1/16 m 1/256 m

(Parte 4 de 8)

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