Sandstone Framework Research Articles

Some Concepts of Cementation and Lithification of Sandstones

Investigation of the processes involved in lithifying sands made up of grains of silicate minerals indicates that crystallization of mineral matter in intergranular pores is only one mechanism which can be classified as cementation. Together with other methods binding grains, there appear to intergradational relations that the term involves mineral authigenesis, recrystallization, and intergranular welding. In effect, cementation and lithification appear to be synonymous terms. Variation in the aspect of cementation is dependent strongly upon the framework of the sand mixture which ranges from sand-grain supported (arenite) to clay-silt matrix supported (wacke). Frameworks intergradational between the two are classified as subwacke. A cement is recognized as compatible if its composition is similar to that of the detrital grain to which it becomes welded. An incompatible cement differs in composition from the grain to which it becomes attached, and adhesion is achieved through surface effects or local grain replacement. A different variety of cementation involves clay minerals attached principally to quartz grains. The attachment is accomplished through coalescence of lattices along a common junction. As diagenetic processes are intensified, each variety of cementation is believed to progress through a series of textural modifications which also may involve compositional changes. Such alterations are part of a sequence for each rock and develop textures which attain culminating stages characteristic of the sandstone framework. Arenite frameworks tend to induce grain overgrowth, which may be followed by recrystallization to produce granular interlocks of bilateral and triple-grain junctions. Wacke frameworks pass through stages of cementation involving reconstitution of clay minerals and recrystallization with authigenic chert into an equilibrium-mineral assemblage. Such assemblages show culminating textures characterized by neoformation of micas and intergrowths with detrital monoc ystalline quartz along margins of such grains. Subwacke frameworks are affected by intense diagenetic processes to develop cementation textures distributed locally in the rock. There may be culminating textures of both arenites and wackes in the same thin section.

Patrick Draw Field, Sweetwater County, Wyoming--An Old Stratigraphic Trap: ABSTRACT

The search for new petroleum reserves can be greatly implemented by a more thorough understanding of why petroleum is trapped where it is. The Draw field, discovered in 1959, started a wave of exploration effort in the Rocky Mountains area to find additional giant stratigraphic traps in the Upper Cretaceous rocks where porous and permeable sandstones pinch out on structural noses. The failure to find another End_Page 1574------------------------------ Draw, despite a widespread exploration effort, signifies that factors not generally considered must have had a dominant influence in the accumulation. These factors are revealed only by examining the geologic history of the area, beginning with the deposition of the reservoir and source rocks and studying the structural attitude of these rocks through time. Although several sandstones are petroleum-productive at Draw, the principal producing zone consists of two sandstone bars at the top of the Almond Formation (Upper Cretaceous). The spatial dimensions, lithologic characteristics, and stratigraphic framework of these bars suggest that they are barrier bar sandstones deposited along the margin of the Lewis sea. These porous and permeable linear barrier bars have a general north-south trend and, updip to the west, grade into impermeable shale and sandstone that were deposited in a swamp and lagoonal environment. A second important productive zone occurs approximately 40 ft. below the top of the Almond Formation. The areal distribution, lithologic nature, and stratigraphic framework of sandstones in this zone suggest that they were deposited as parts of a tidal delta in a lagoon. Each of the three main productive sandstones has a different oil-water contact. The geologic history of the Draw area shows that, by the beginning of the time of deposition of the Lance Formation (Upper Cretaceous), conditions were favorable for petroleum accumulation. The reservoir sandstones had 1,200 ft. of overburden and several million years had elapsed since the reservoir sandstones were deposited. An early trap was formed where these sandstones were warped over an east-plunging structural nose, and early migration of petroleum produced a large accumulation a few miles south of the present field. When the present Wamsutter arch came into existence in post-early Eocene time, the first trap was opened and the accumulation spilled northward to be trapped at the present location of Draw field. The search for more Patrick Draws must include more than an analysis of present structure and potential reservoir rock. The time of formation of the trap, the structural modification of the trap through time, and the associated origin and migration problems are hidden factors that play the dominate role in formation of a large petroleum accumulation. Exploration geologists must know more about the regional framework of sedimentation and the cause and effect of incipient structural development in depositional areas, and understand how these factors relate to the geologic history of a region. End_of_Article - Last_Page 1575------------

Practical Petrographic Classification of Limestones

Limestones are divisible into eleven basic types, which are relatively easy to recognize both in the laboratory and in the field. These rocks are made up of three constituents: (1) allochems, evidently transported or otherwise differentiated carbonate bodies; (2) 1-4-micron microcrystalline calcite ooze matrix, and (3) coarser and clearer sparry calcite, which in most rocks forms as a simple pore-filling cement (like the calcite cement in a quartz sandstone), and only uncommonly forms by recrystallization. Only four types of allochems are volumetrically important in limestones: (a) intraclasts (reworked fragments of penecontemporaneous carbonate sediment), (b) oolites, (c) fossils, and (d) pellets (rounded aggregates of microcrystalline calcite averaging .04-.10 mm.). All chems provide the structural framework of limestones, just as sand grains provide the structural framework of sandstones; microcrystalline calcite and sparry calcite are analogous with the clay matrix and chemical cement of sandstones. A triangular diagram showing the relative proportions of allochems, calcite ooze matrix, and sparry calcite cement is used to define three major limestone families. Family I consists of abundant allochems cemented by sparry calcite; these are the cleanly washed limestones, analogous with well sorted, clay-free sandstones and similarly formed in loci of vigorous currents. Family II consists of variable amounts of allochems embedded in a microcrystalline ooze matrix; these are the poorly washed limestones that are analogous with clayey, poorly sorted sandstones, and form in loci of ineffective currents. Family III limestones consist almost entirely of calcite ooze, hence are analogous with terrigenous claystones. Just as clayey versus non-clayey sandstones can be divided mineralogically into orthoquartzites, arkoses, and graywackes, similarly the first two limestone families are subdivided by considering the nature of the allochems. Family I includes respectively intrasparite, oosparite, biosparite, and pelsparite; family II includes intramicrite and oomicrite (both rare), biomicrite, and pelmicrite. Family III includes homogeneous ooze (micrite), and disturbed ooze with irregular openings filled with spar (dismicrite). Rocks made up largely of organisms in growth position are considered as a separate family IV (biolithite). Properties and mode of formation of each of these types are discussed briefly. Content of admixed terrigenous material or dolomite is shown by additional symbols; pure dolomites are classified on allochem content and crystal size. Recrystallization in limestone is believed to be locally abundant but of over-all minor importance. Among several types of recrystallization, that in which a former microcrystalline ooze matrix recrystallizes to 5-15-micron microspar is considered most common. The term calclithite is suggested for the terrigenous carbonate rocks, e.g., limestone conglomerates End_Page 1------------------------------ or sandstones made up of material eroded from outcrops of considerably older lithified-carbonate formations exposed in an uplifted source land.