Peter A. Scholle

Carbonate Depositional Environments

For more than 100 years geologists have been ex amining and describing modern sediments with an eye toward using characteristic features to aid in the interpretation of depositional settings of ancient strata. This field of interest developed particularly during the 1950s and 1960s with the creation of detailed models for modern carbonate deposition in Florida, the Bahamas, Cuba, the Persian Gulf, Belize, Pacific atolls, the Great Barrier Reef and other areas. An understanding of the depositional environments of these modern models, coupled with increased understanding of diagenetic effects, has led to vastly improved interpretations of ancient limestones. Such models also improved the “predictability” of many carbonate reservoir rocks. In spite of the great strides made in our knowledge about carbonate depositional environments, their characteristic features have never been synthesized in a single work. Although excellent textbooks exist which describe some aspects of the interpretation of both ancient strata and modern sediments, systematic treatment of the entire subject is available only in the primary literature. This book is an attempt to bring together this widely disseminated literature. The volume is specifically designed for use by the non—specialist-the petroleum geologist or field geologist—who needs to use carbonate depositional environments in facies reconstructions and environmental interpretations. Yet it is hoped that the book will also serve as a valuable reference for the specialist or advanced graduate student. Toward that purpose, the book is extensively illustrated with color diagrams and photographs of sedimentary structures and facies assemblages. The text focuses on the recognition of depositional environments rather than on the hydrodynamic mechanisms of sediment movement.

Chalk Diagenesis and Its Relation to Petroleum Exploration: Oil from Chalks, a Modern Miracle?

Chalks consist largely of stable low-magnesium calcite. Thus, they undergo diagenetic alteration different from that of more widely studied aragonite and high-magnesium calcite-bearing, shallow-marine carbonate deposits. Examination of outcrop and subsurface samples of chalks from the North Sea, onshore Europe, the Scotian Shelf, Gulf Coast, and the U.S. Western Interior indicates that chalks undergo significant diagenetic changes during their postdepositional history. Scanning-electron microscopy, light microscopy, oxygen-isotopic analysis, and trace-element analysis outline the major factors that control the patterns of chalk alteration. The major mechanism of chalk cementation is pressure solution and local reprecipitation. Although small variations in initial grain size, faunal composition, or clay content can lead to significant bed-to-bed variations in cementation, overall patterns of chalk diagenesis appear to be related to two main factors: (1) maximum depth of burial, and (2) pore-water chemistry. With a few notable exceptions, the porosity (and permeability) of chalks decreases as a direct function of burial depth. The exceptions include cases where: (1) oil entered the rock, reducing or terminating carbonate reactions; (2) chalks are overpressured and therefore are not subject to the normal grain-to-grain stresses expected at those depths; and (3) tectonic stresses increase solution and cementation. In areas here fresh water entered the pores before major burial, chalks show a much steeper gradient of porosity loss versus burial depth as compared with regions where marine pore fluids were retained. Under normal circumstances, a typical nannofossil chalk ooze will have 70% porosity at the sediment-water interface. At a depth of 1 km, porosity will be reduced to about 35%; at 2 km, to about 15%; and at 3 km, to essentially 0. Thus, one observes progressive lithification of chalks (and their isotopic alteration) as one moves downhole or toward areas of greater burial. Petrophysical and isotopic studies can predict maximum depths of burial, paleogeothermal gradients, and proximity to zones of deformation. In areas such as the Ekofisk field in the North Sea, however, major quantities of oil are produced from chalks having as much as 40% porosity (largely primary) at depths greater than 3 km. This appears to be related largely to the widespread overpressuring of the Central graben in that area. Other such areas of anomalous porosity in thick chalk sections should be detectable by seismic methods. Significant hydrocarbon production from chalks can occur in three major settings: (1) overpressured or oil-saturated zones where these phenomena were initiated early in the subsidence history (e.g., the North Sea); (2) areas where chalks never have been buried deeply (e.g., the Scotian Shelf); and (3) cemented and fractured chalks in several possible settings (e.g., the Gulf Coast).

Aspects of Diagenesis

There are a number of gaping holes in accumulated knowledge within the discipline of sedimentology. Perhaps one of the largest holes has been the general subject of diagenesis in clastic rocks. It was therefore fortuitous that two symposia covering various aspects of diagenesis (mainly in clastics) were presented a year apart in different parts of the country but with the same motivation – to contribute to the closing of that knowledge gap. Sedimentologists now have a fairly good idea of the what and the how of sediment deposition. What happens after the sediments are lithified has frequently been ignored. It was the aim of both editors of this publication to approach the subject from two different viewpoints. Schluger directed a symposium which looked mainly at clastic reservoirs, and Scholle presented a symposium which examined various aspects of paleotemperature control of diagenesis.

Deposition of resedimented sandstone beds in the Pico Formation, Ventura Basin, California, as interpreted from magnetic fabric measurements

Settling velocity distributions and magnetic fabrics in sediments from the Pico Formation were studied in order to determine the relationships between these properties and the observed sedimentary structures, and to evaluate the processes of deposition of turbidites. Three basic types of settling velocity frequency distributions were recognized: P1 profile, a low, flat distribution pattern indicating very poor sorting; P2 profile, a distribution which shows a distinctive mode; and P3 profile, a slope-shaped pattern composed predominantly of fine materials. P1 was found in the graded and massive divisions of turbidites; P2 was found in the lower division of horizontal stratification and the ripple stratification division; P3 was found in the uppermost divisions. Comparison of these patterns with previous results from modern sediments reveals that P2 and P3 are similar to patterns found in fluvial environments, whereas P1 was quite rare in “normal” current- or wave-formed deposits. The uniqueness of the graded and massive division is also evident in the results of the magnetic analysis. Although the magnetic fabrics in the upper parts of turbidites show similarity to other current-formed fabrics, the magnetic fabrics of the graded and massive divisions are quite different. The magnetic fabrics in the graded and massive divisions are characterized by (1) the presence of the current-normal orientation, and (2) less foliated and in-homogeneous fabrics which are indicated by high imbrication and q -value as well as large standard deviations of q -value and K max directions. Comparison with results from modern sediments indicates that fabric characteristics in the sediments of the graded and massive divisions are best explained by a combination of (1) an orientation mechanism related to layer by layer grain collision in a highly concentrated flow and (2) an orientation mechanism related to the suspension of grains in a viscous flow. This evidence indicates that a highly concentrated and partly viscous basal flow in turbidity currents may be responsible for the deposition of the lower part of the graded division and the massive division, whereas a more diluted flow may be responsible for the deposition of the upper divisions.

Southern British Honduras: Lagoonal Coccolith Ooze

ABSTRACT The British Honduras carbonate depositional province contains a relatively narrow, deep, back-reef lagoon in which Recent fine-grained carbonate sediments are accumulating. Examination of the finer fractions of these lagoonal muds by optical and scanning electron microscopy reveals that a large proportion of the sediment (as much as 20 percent) is composed of coccoliths and coccolith fragments. Eight nannoplankton species have been described. Matthews (1965, 1966) failed to describe coccoliths and other fine-grained constituents in his study of British Honduran muds because he did not petrographically examine grains smaller than 20 microns. Thus comparisons between his data and the finer muds of Florida. the Bahamas. and other areas must be undertaken with caution because of the differences in grain sizes examined. The lagoonal, coccolith-rich carbonate muds of British Honduras which surround coralgal pinnacle reefs provide an excellent Recent analog for a number of ancient carbonates, including the Solnhofen Limestone. They also indicate that coccolith-rich sediments need not indicate deep-water deposition or unrestricted circulation far from land masses or major hydrographic barriers.

Origin of Bimodal Sands in Some Modern Environments

Grain settling velocity distributions of sediments from modern fluvial channel, eolian barchan dune, beach, and tidal flat environments of the United States and Mexico were analyzed using a photo-extinction settling tube. The results revealed the occurrence of bimodal distributions in several specific places: (1) trough cross stratification in fluvial channels, (2) the lower slip face, lower stoss side, and crest of barchan dunes, (3) eolian mega-ripples, (4) lower beach foreshores, (5) beach storm layers, and (6) inner parts of tidal fiats. Possible sorting mechanisms for generating bimodal characteristics of these deposits are suggested and are classified into three categories: a) Mixing of two sorting processes which have different sorting tendencies. For example, mixing of avalanche sorting and projection sorting on the dune slip face is responsible for bimodal sediments of types (1) and (2). b) Special hydraulic circumstances yielding high mobility of the coarse fraction; type (3) and (4) bimodal sands are examples. c) Unusual transportation events. Storm transportation of a coarse fraction into ordinarily coarse-sediment-free environments may be responsible for type (5) and (6) distributions. This study has demonstrated that bimodal distributions of grain settling velocity of sediments could be interpreted by simple and basic sorting processes.

Radiaxial Fibrous Calcite as Early-Burial, Open-System Cement: Isotopic Evidence from Permian of China: ABSTRACT

The Nanpanjiang basin of south China occupies about 100,000 km2 in southern Guizhou and eastern Yunnan Provinces and northwestern Guangxi Autonomous Region. The basin contains a thick Paleozoic carbonate sequence overlain by about 3,000 m of Triassic basinal deposits. Permian carbonate rocks comprise a large portion of the Paleozoic strata and form several platforms separated by basins containing dark, thin-bedded limestones, siliceous shales, and cherts. The platform margins are rimmed by sponge or algal reefs. Radiaxial fibrous calcite (RFC) is the most abundant cement in very coarse sponge or algal debris of Upper Permian reef and fore-reef sediments exposed along the western margin of the Nanpanjiang basin. Small volumes of syndepositional cements, interpreted to have been fibrous magnesian calcites and botryoidal aragonite, predate RFC. Coarse, blocky burial calcite postdates RFC. Evidence that RFC was precipitated during sediment deposition was not found. RFC occurs as isopach layers up to 15 mm thick and exhibits white, gray, and black bands about 1 mm wide. The presence of microdolomite inclusions in these cements indicates that they were originally magnesian calcites. ^dgr18O of RFC cements are more positive than any of the earlier or later components of the reef and fore- eef facies. Analyses of successive bands reveals the most positive ^dgr18O near the center of the isopach layers. ^dgr13C of successive bands reveals generally more negative values toward the centers of layers. RFC layers are interpreted to have precipitated during early burial of the platform margin while reef and fore-reef sediments were in communication with seawater. Cement layers recorded isotopic characteristics of seawater as platform-edge sediments subsided through the water column at the basin margin. ^dgr18O of successive bands records cooler water at depth in the basin followed by geothermal warming. ^dgr13C records increased incorporation of light carbon as the platform subsided through the oxygen minimum zone, followed by a return to normal values at depth. These data and interpretations suggest RFC layers precipitated very slowly during time spans commensurate with those of subsiding platforms (millions of years). Isotopic characteristics of RFC may not reflect shallow seawater. Rather, they may reflect burial environments where ^dgr18O is affected by cooler water and ^dgr13C is affected by biologic activity. End_of_Article - Last_Page 261------------

Design and Calibration of a Photo-extinction Settling Tube for Grain Size Analysis

ABSTRACT A new type of settling tube has been designed for analysis of settling velocity distribitions in sediments having diameters ranging from clay to gravel size. This tube employs two different sample introduction devices for coarse- and fine-grained samples and also features an optional sample recovery system. The output of the tubes photocell has been calibrated in terms of theoretical settling velocities of glass spheres using glass microbeads and employing simple photo-extinction theory. The results agree with those predicted by theory to within about 2% and are reproducible in most cases to within 2%. The main reason for this improved precision and accuracy is the high sensitivity of the system which allows analysis of very small amounts of sample and thus reduces the effects of gra n interactions to a minimum.

Subaerial Exposure and Erosion in Capitan Reef (Permian), Guadalupe Mountains, New Mexico: ABSTRACT

A prominent subaerial exposure surface has been identified within the Capitan reef in Rattlesnake Canyon. Equivalent to the middle Yates interval in the back reef, the exposure surface records planation of the reef during a sea level drop to below the shelf edge. This is the first exposure surface recorded within the reef, although several are known within the back reef facies. The surface is quite planar and can be traced shelfward for nearly 1 km before it is lost in near-back reef grainstones. The surface does not appear to correlate directly with one of the Yates sandstone beds. Channels cut into the surface have a maximum relief of nearly 2 m. Gypsum molds (now calcite filled) and mud cracks are found in the channel-filling sands. In nonchannel areas, a thin (1 to 5-cm) zone of reddened pebbles is sometimes present. Although the exposure horizon has only a thin sediment veneer, the extensive truncation of the underlying beds implies significant sediment transport across the surface. This is presumably reflected in one of several prominent clastic lowstand wedges found in the Bell Canyon Formation of the Delaware basin.

Porosity Relations in Chalk Reservoirs: ABSTRACT

Oil and gas reservoirs in chalks of the Gulf Coast, Denver basin, and North Sea show similar porosity relations. Most of the storage capacity in the three areas comes from the preservation of primary porosity. Normally, the high initial porosity (60 to 75%) of chalks is progressively lost during burial owing to mechanical and chemical compaction effects. Thus, in many areas of the Gulf Coast and the Western Interior, paleoburial depths of about 1,000 to 1,500 m (3,300 to 5,000 ft) form an economic lower limit for exploration because primary porosity has been drastically reduced at greater depths. Three factors can strongly influence this relation of porosity and burial depth. First, fracturing can greatly improve the effective permeabilities of chalk reservoirs. Fracturing related to gentle flexuring, salt-dome tectonics, or fault zones has a major influence on the reservoir characteristics of North Sea and Gulf Coast fields and may be involved in Western Interior fields as well. Second, abnormally high pore-fluid pressures (geopressures) reduce or completely halt mechanical and chemical compaction and thus aid in the preservation of primary porosity. In the North Sea and offshore Louisiana, geopressuring has allowed preservation of as much as 40% porosity at depths of greater than 3,000 m (10,000 ft). Finally, early formation of biogenic methane (from bacterial decomposition f organic matter contained within the chalks) or early introduction of migrated hydrocarbons to the point of virtual oil or gas saturation (as in some North Sea chalks) may also be instrumental in porosity preservation during burial. The porosity relations in chalks, although fairly complex, are far simpler than those typically seen in shallow-water limestones. Thus, based on relatively sparse data, reservoir properties and petroleum potential of chalks can be reliably predicted throughout large areas. End_of_Article - Last_Page 522------------