Detailed Petrographic Descriptions, McKittrick Field, Core 418 Page 1

 

 

The following section provides detailed petrographic descriptions of specimens from selected depths. The data include optical and electron analysis from thin sections and grain mounts. Petrographic observation and optical photomicrographs were performed on a Nikon petrographic microscope; most descriptions were done at 40X or 100X magnification. Electron images were acquired on a JEOL JSM-820 SEM with a Tracor-Northern TN5502 Series II analyzer. EDS analyses were performed using manufacturer supplied intensities from calibration standards, and normallized to 100 wt%. Margin of error for analyses is approximately 2% relative for concentrations greater than 10 wt%, 10% relative for concentrations from 5% to 10%, and 50% relative for concentrations between l% to 5% by weight. Grain Mounts were coated with 35 manometers of gold in a Hummner VII sputter coater by Anatech, Ltd. .

 

McKittrick rocks appear to be two lithological types on basis of hydrocarbon occurrence:

 

1) Fine grained material with uniformly distributed hydrocarbons which impart a brownish cast to the section. Vertical and horizontally cut sections the same appearance; suggesting a very Similar 3-dimensional structure.

 

2) Pods or segregations of hydrocarbons with varve-like structure when viewed in section cut perpendicular to bedding. In sections cut parallel to beds, they appear as scattered, amorphous segregations.

 

418-1186.5 This specimen was chosen because it is opal A with no tendency to vertical fracturing. The specimens are labelled A and B; they are perpendicular to each other, but in preparation of the sections, the core orientation was not apparent. In thin section, the rock is fine grained material with hydrocarbon so abundant that it obscures all other features. This rock is so incompetent that alumina polishing compound is literally pushed into the section, and could not be removed by ultrasonic agitation. The hydrocarbons are more abundant in some horizontal beds than others; 35mm slide 1 shows a vertical segregation of hydrocarbon in a mini-fracture, perpendicular to bedding. Photo 1, st 40X is transmitted plane polarized light. Listed as high quality opal A, So is 73%. Note in particular: the center of the photo has spherical silicious microfossils, calcareous tests, and pyritized (formerly calcareous test) microfossil at edge of photo. hydrocarbon regions are dark and soft; alumina polishing compound is imbedded and can't be removed. Some replacement suggests reducing fluids (from presence of hydrocarbons).

 

Grain mounts are shown in the 6000 series SEM photos. Photo 6000 is 1000X SEI showing the fragments of detrital microfossils, as well as some carbonate (center). A qualitative scan gave approx. 88% Silica, 4% Alumina, 3% CaO, 2% FeO. The small amorphous grain above the carbonate analyses with proportions of Mg Si(2.5); it is an oxide. Photo 6001 shows silica being reprecipitated around the holes in the diatoms. 6002 is the assorted detrital fragments. The euhedral lath in center is calcite. 6003 is a higher magnification (5000X) of the plates, showing different pore sizes. They appear to be in various states of dissolution. Photo 6004 is a low magnification (150X) image; the large spherical silica microfossil in center at 500X is seen photo 6005.

 

418-1228.8 This specimen was chosen because it is opal A, with 68.1% So with low clay. The structure appears random in both directions, the hydrocarbons are thinly and uniformly disseminated throughout the entire section. In this case, the pyrite appears to be spherical but not framboidal - 35mm slides 3-4 suggest that it is replacing the diatoms, which are not abundant, but retain their original structure. The diatoms are important as "porosity " for collecting hydrocarbons; if pyritization occurs first, this may be important. Photo 2: plane polarized reflected light, 40X, 68.1% So. Note that even at this level, hydrocarbons accumulate along fractures. There is also non-uniform distribution of hydrocarbon which is also apparent in hand specimen. Dark regions are hydrocarbon, brighter matrix. Bright grains are polishing compound, stuck in hydrocarbon rich regions.

 

418-1230.4 This specimen was chosen because it was described as high clay Opal A, 35.2 So contrasting from 2 ft above which has 68.1%. There is a density difference apparent in hand specimen. The 35mm photo is a 40x transmitted plane polarized light image. This also shows hydrocarbon filled fractures although hydrocarbon content is much lower.

 

418-1399.5 has a permeability of 46 md. The section appears to be full of fractures; the vertical section also has fractures which parallel the bedding. There are small colorless crystals in the pores; they have parallel extinction. The high porosity is probably due to these pores, possibly secondary porosity.

 

418-1401.5 This sample is referred to as "Carbonate Hard Streak" in the preliminary descriptions. Under the petrographic microscope, the sections appear the same, regardless of cut. Although the samples were impregnated with blue stained epoxy, these sections have blue epoxy visible only in the largest fractures. In thin section, the sample appears to be mostly fine grained carbonate. A few regions of reddish brown stain are apparent (hydrocarbons), and a few angular silica fragments. Dark specks (pyrite), usually <50u, are visible; they make up less than 1% of the rock. One fragment of plagioclase was seen, identified by the albite twinning. Isolated dark pore spaces, either produced by plucking of detrital grains, or unable to be epoxy penetrated, are present. Clearly, some of the dark pores are true porosity - they are partially filled by rhombic-faced carbonate crystals which have clearly crystallized into open pore space. The detrital grains, pores, and microfossils appear to be frequently replaced by carbonate - they are highly birefringent under crossed polars, optically continuous, rhombehedral, and several hundred microns or less. Sometimes all that remains is relic textures of diatoms, totally replaced by carbonate. A region inside a fracture was circled- there is a bright white grain inside of the vein which analyzed as coarse grained carbonate. I also see small framboidal pyrite everywhere. There appears to be some variation along the edge of the fracture.

 

418-1424.5 This is listed as the first A/CT transitional and I think it is a good one to look at to see the changes in the silica phases. The optical photomicrograph shows an extremely fine grained, uniform appearing section with abundant microfossils. There is some replacement by pyrite of detrital angular silica grains (the comer of one is pyrite). I also see pyrite filling the pores in fossils although sometimes pyrite also replaces the silica tests and leaves behind pores. There are tiny little doughnut shaped fossils.

 

418-1522.6 The series 7000 are grain mounts from this specimen. This is "transitional" by Jack's description. Note that at 150X magnification, the structures of the detrital material are not visible. The authigenic feldspars and the blebs" of silica indicate that recrystallization is extensive already at this depth. This one appears to have a very interesting chemistry and unusual phases.

In photo 7000, at 150X, the structures of the small silica fragments are not visible, also the large microfossils outlines are still apparent, but have been modified. At 500X (photo 7001), compare this to the large radiolarian (?) in photo 6005. The pores in the microfossil are gone, although the silica "rim" around the exterior of the microfossil is still apparent. Photo 7003, at 1000X, little of the fragmental character of the original detrital material is apparent. Note at the center top, is a unshaped indentation which is probably caused by silica spines which were more resistant to dissolution, and have left channels in the structure after neocrystallization. 7004 is 500X of dissolving detrital garbage. Just above center to the left is a small authigenic albite grain, seen in higher magnification in photo 7005. Note that it appears in a vug, it has obviously precipitated after the removal of some material. Clearly authigenic albite, it analyzes as:

Na.98 Ca.02 Al Si3 08.

Although the grain is euhedral, the surface is rough and the crystal faces are not perfect.

 

Semi-quantitative EDS compositional analyses are taken from points on photo 7006, and are listed in the table of analyses. The first two analyses are the bottom of the albite grain. The grain directly below with stepped morphology and what appear to be crystal faces consists of Si, Al, and Ca. I have consistently gotten a similar analysis on many grains. This may actually be zeolite, although the Al seems too low and Ca too high. If this is really zeolite, that is very important because zeolites can "trap" organic molecules. Also, if porosity is taken from neutron logs, the zeolites (15 to 18% H(2)0 by weight) will give a seriously overly high porosity. Three analyses (#3,4,5) were done on the same grain and were all slightly different, but basically: Ca Al Si(4-5) (with variable minor Na, probably substituting for Ca). Sometimes the Al is extremely low, AI:SI is 1:10. The amorphous, spherical blebs are (by wt) 80-85% SiO2, 10% Al(2)0(3), with noise levels of everything else. The flat plates are (Fe,Mg) Al Si(5) in composition. The compositions are so variable, and so silicious, I can't help but imagine they are actual phases which are coprecipitated or contaminated with silica.

 

This 1000X SEI (7008) shows a distinct change in morphology from 1187 ft. A authigenic albite crystal occurs in an open pore. "Channels" of indentations in the silica persist - these may be molds made from silica microfossil spines. The large flat particle in the center was analyzed and had a variable composition; Si:Al was 5:1 and was high in Ti and Fe (might it be a volcanic glass shard?). Another region was analyzed from photo 7009 and are listed in the table of analyses. The authigenic feldspars and the "blebs" of recrystallized silica indicate that recrystallization is extensive already at 1522. This one appears to have a very interesting chemistry and unusual phases.

 

418-1526.7 The hydrocarbons/staining in the section cut vertical to bedding one seems to be quite uniform, not well segregated at all. It also appears to be very clayey-little texture, birefringence, or structure, other than abundant microfossils. Some of, the small grains look like illite-with lath morphology and high birefringence, but might be carbonate fossils. In reflected light, the pyrite is framboidal, also seems to be replacement or amorphous grains. Some detrital grains (quartz?) also appear under reflected light - they are up to .1 The horizontally cut section looks very much the same as the vertical section suggesting the same 3-dimensional structure. On the 100X 35 mm photo, are fossils and detrital grains. Spherical microfossils are filled with black material (not blue) and is probably hydrocarbon. The field is dark under crossed polars, suggesting silica and not carbonate.

 

418-1631.5 This specimen is very different from most others; it contains pyritized fossils and gypsum vein filling. In the horizontal section, abundant reddish-brown stain is everywhere, it may be low concentration of hydrocarbon. There are patches of fine grained carbonate; coiled fossils have been replaced by pyrite which suggests a change in fluid composition and oxidation conditions. Some fossils are still carbonate. Also, 35 mm photo of hydrocarbon accumulation along fracture.In the horizontally cut section, the fracture is lined with colorless, fibrous, low birefringent material. SEM/EDS indicates it is Ca-S; the birefingence suggests gypsum. Note that this corresponds to a temperature less than 42 degrees; Deer, Howie, Zussman note that the transition to anhydrite occurs at lower temperatures in the presence of NaCl, so this seems to indicate a low temperature assemblage. A change in lithology/texture is apparent in this one, part is fine grained "varve-like" structure; it grades to coarser detrital grains with more amorphous hydrocarbon material. All veins/fractures show some kind of alteration along them.

 

In SEM, reconnaissance indicated a fairly simple mineralogy. The minerals found are 1) Feldspar, probably anorthoclase or oligoclase (based on EDS spectrum) as 100æ grains. Also saw K-feldspar, with a small Na component. The matrix is silica, with a little bit of S in the signal. When the UltraThin window is inserted, I see no carbon peak, in spite of the fact that is has been carbon coated. The sulfur might be important with respect to grade of silicization; most other silica regions show small amounts of Al. Vein filling is gypsum or anhydrite. Another dark region using backscatter reconnaissance which I thought was hydrocarbon gave an analysis 62% SiO2, 31% S. The matrix is predominantly silica with feldspar (k-feldspar, albite, and plagioclase). SEM Photo 1636 shows the texture. Photo 1637, the corresponding SEI, seem to illustrate a different texture in the hydrocarbon rich silica, as opposed to the open porosity as white circles, due to chargeup. This will give us a tool for evaluating open porosity vs. hydrocarbon filled porosity. Some of the detrital feldspar grains appear to be re-equilibrating; the large feldspar grain shown in photo 1638 has albite at the dark corner (slight Ca), with K feldspar (no Na or Ca) in the bright region. K feldspar also occurs with pyrite inclusions.

 

Horizons in this section go from fossil-rich to finer grained. Many of the fossils are still calcium carbonate (SEM photo 1639, light gray clusters of grains), but some have been replaced by pyrite (SEM photo 1641). In this case, the open chambers in the coiled shell are filled with gypsum. Veins in this sample are also filled with gypsum. Some of the shells gave mixed calcite/silica signals although it us not clear at this point if the shells are silicified or it is just matrix contamination. Note also the hydrocarbon region in SEM photo 1639 - this appears to be a diatom filled with hydrocarbon. The silica in this structure is free from aluminum - which is often present in the "matrix" regions.

 

418-1634.8 This sample's porosity, permeability, saturation test shows high So, but fluorescence does not. The vertically cut section is very dark with staining everywhere. The structure alternates between the varve-like, very dark segregations; abundant fossil horizons; and fine-grained "matrix" within several mm. Rather than pyrite, the fossils are silicified-some birefringence shows in the structure. It may be incomplete (from carbonate) or a non-silica phase. In the horizontally cut section, The same structures are present, even in this orientation

 

418-1681.4 Although listed as pure CT, under crossed polars, this looks like carbonate (it is not a dark field) and fossils are carbonate.

 

418-1723.7 The section cut perpendicular to the bedding shows lamellae and bedding clearly, has dark fractures in random directions, as well as pods of blue epoxy. The staining is oriented in elongated, wavy pods which are subparallel to bedding planes. Small, spherical, black blebs are common. Much fracturing with fractures intersecting and truncating each other. There is extensive random fracturing in this sample; it is filled with blue epoxy and hydrocarbons. Some of the fractures offset the pods. Also, a vug with crystals in a hydrocarbon filled vein indicates that hydrocarbon filling is not a polishing artifact. The fracturing in the sample clearly will increase permeability. Circled region is dark-filled fractures; some fractures in other areas are epoxy filled. The matrix is very dirty looking but also exhibits high order birefringence, which suggests that there is not a lot of clay in it. The horizontal section is very dark looking and has large (mm) size regions of staining. It is non-uniform, heavier in some regions than in others. 10 to 20æ spherical blebs (Hydrocarbons?) are scattered throughout the section. The outlines of the detrital grains are very hard to discern at this point, probably due to the fine grained matrix. When polars are crossed, small (20æ) birefringent grain are seen going in and out of extinction (carbonate?). It is very difficult to see the particles in this one, even at 200X. Backscattered images show a darker region along fractures, which also contains carbonate rhombs. The series 9000 SEM photos are from the grain mounts. They show primarily dissolution of silica microfossils, and secondary (?) porosity.

418-1753 Photo 8000 is a grain mount of 1753.5, It has virtually no Al in the area scan - very unusual for these rocks.

 

418-1754.5 This is referred to as "silty" in the original descriptions. The section cut perp. to bedding shows dark segregations subparallel to bedding, as well as large (500-1000æ) fractures. Section appears predominantly isotropic when rotated under crossed polars; assumed to be due to the presence of chert. The dark brownish staining is everywhere. Individual grains are difficult to discern; small birefringent grains are assumed to be carbonate. Note- one vein has polishing compound in it. Fossils are very easy to recognize in the section cut parallel to bedding. They are grayish, but not isotropic. They look "frothy" and white in reflected light, and are probably carbonate. Like the other cut from this section, the section appears very dark under crossed polars, and very small (25æ) somewhat birefringent grains can be seen. Grain mounts are shown in photos 8050 and 8051. Photo 8051 has fossil fragments, still recognizable, which is very unusual for this depth. Most have re-precipitated by now, so there must be a barrier to the fluids at this limited horizon. The fragments analyze qualitatively as Ca+Si. Several analyses were done on Si/Ca phases; they did not have a composition which was readily recognizable as a mineral phase.

 

418-1756.5 This is described as being similar to Pioneer. The vertically cut section is all reddish stained, and alternates between lamellae of fine grained chert to silty, hydrocarbon segregations, to silty with detrital grains. The hydrocarbon looks frothy, dark, massive. The hydrocarbons are totally excluded from the chert lamella, which is cut by a series of pyrite framboids.

 

415-4827 Deeper specimens are from well 415, and a grain mount examination is all that has been performed on this specimen. The photos are the 9050 series. The images indicate a highly porous rock, with late stage precipitates in open pores. The precipitates have nucleated on earlier silica phases, and may be a result of precipitation from brines as the specimen dried, or they may be from drilling fluids, or both. Photo 9050 shows gypsum or anhydrite (large stepped grain, small stepped grain). Also present is NaCl, and silica phases with small amounts of Al visible in the EDS spectrum. Also present are filaments which are too small to get a good analysis but appear to be a Ca phase. These analyses are also listed in the table of analyses. Grains in photo 9053 have been analyzed and several analyses are listed in the table of analyses. They all appear to have a high Si/Al and variable Ca, Na, Fe, Mg, and K. These may be either zeolite or volcanic shards.