Robert L. Freed

Geopressured Shale and Sealing Effect of Smectite to Illite Transition

Ten shale samples of Frio and Vicksburg (Tertiary) overpressured shales from Brazoria and Hidalgo Counties, Texas, were examined by transmission electron microscopy (TEM) techniques. Initially, illite and smectite (with some mixed-layer component?) are present in subparallel domains, with smectite packets dominating and providing abundant dislocations for ion transport, supplying avenues that contribute to local permeability. As the transition proceeds, illite packets grow within a shrinking matrix of smectite, dislocations decrease, and pathways for ion transport are restricted. We suggest this development of illite packets and diminished ion transport causes loss of local permeability and a rise in the fluid pressure gradient. As illite packets coalesce, local permeabil ty is increasingly lost; hydraulic continuity with the surface is further restricted and a more efficient geopressure seal is formed. In both Hidalgo and Brazoria Counties, the base of normal hydropressure (0.465 psi/ft or 10.5 kPa/m) coincides with the depth of onset of the smectite-to-illite transition. Geopressure greater than 0.7 psi/ft (15.8 kPa/m) occurs at depths corresponding to 50% illite packets in Brazoria County and 70% in Hidalgo County; in both cases, 0.7 psi/ft (15.8 kPa/m) geopressure occurs prior to the depth at which the transition ceases. These relationships suggest that if the smectite-to-illite transition begins and proceeds to completion, the development of coalescing illite packets during diagenesis will lead to increasing geopressure, with the actual values of illite proportion and corresponding fluid pressure gradient determined by the local geologic setting.

Diagenesis and the Formation of Authigenic Illite-Rich I/S Crystals in Gulf Coast Shales: Tem Study of Clay Separates

ABSTRACT Shale samples ranging in depth from 305 m to 3658 m (Miocene to Late Cretaceous) from a single well in DeWitt County, Texas were examined by x-ray diffraction (XRD), scanning-transmission electron microscopy (STEM), and analytical electron microscopy (AEM) techniques to detail the mineralogical, textural, and chemical changes during diagenesis. XRD data suggest that from the surface down to 2134 m, illite/smectite (I/S) is < 50% illite. AEM data show that this smectite-rich I/S contains more K than Na or Ca, suggesting that at least some K exchange with pore fluids occurred well before the transition to illite-rich I/S, and implying a lesser role for detrital K-feldspar and mica in the transition. In addition, direct STEM/AEM observations of separates show that these smectite-rich amples contain equidimensional, euhedral, authigenic illite-rich I/S crystallites at depths as shallow as 610 m; electron-diffraction yields single-crystal patterns. However, such crystallites are rare compared with common anhedral flakes; electron-diffraction patterns for anhedral masses show powder rings, indicating random orientation of layers. Al/Si ratios for anhedral material imply a smectite-like high Si content for tetrahedral sites; Al/Si ratios for the euhedral crystallites suggest an illite-like lower Si content for tetrahedral sites. Between 2134 m and 2438 m (83° to 88°C), the smectite to illite (S-I) transition abruptly occurs, such that at 2438 m and deeper the I/S content remains constant at 75-90% illite. Direct STEM/AEM observations of separates of these illite- ich samples show abundant, equidimensional, euhedral, authigenic, illite-rich I/S crystals; electron-diffraction shows single-crystal patterns. XRD data show that mixed-layer I/S is dominantly R1 ordered. We interpret these data to suggest that, initially, anhedral smectite-rich material is dominant with significant K in exchangeable cation positions; rare illite-rich I/S crystals form in the early stages of diagenesis in Gulf Coast pelitic rocks. During the abrupt S-I transition, the original random orientation of smectite layers is lost and a coherent arrangement of illite packets is formed. The S-I transition proceeds by dissolution of smectite-rich material and utilization, at least in part, of exchangeable K to develop abundant, euhedral, dominantly R1-ordered, illite-rich I/S crystals. The development of illite-rich I/S crystals proceeds via an Ostwald-step-rule process: dissolution of metastable smectite-rich I/S and growth of abundant, euhedral, R1-ordered, metastable illite-ri h I/S crystals.

Shale Mineralogy and Burial Diagenesis of Frio and Vicksburg Formations in Two Geopressured Wells, Mcallen Ranch Area, Hidalgo County, Texas

ABSTRACT Thirty-six shale samples ranging in depth from 1,454 ft. to 13,430 ft. from Shell Oil Company No. 1 Dixie Mortgage Loan well and 33 shale samples ranging in depth from 2,183 ft. to 13,632 ft. from Shell Oil/Delhi-Taylor Oil Corporation No. 3 A.A. McAllen well were examined by x-ray techniques to determine the mineralogical parameters of the geopressured zone in the Vicksburg Fairway. Both wells have the same weight-percent trends with depth for the mineralogy: quartz, calcite, total clay, and potassium feldspar are constant; plagioclase feldspar gradually increases; kaolinite increases; discrete illite decreases; total mixed-layer illite-smectite (I/S) decreases; illite in mixed-layer I/S increases; and smectite in mixed-layer I/S decreases. Chlorite is found only in the geopressured zone of each well. Significant diagenetic changes begin at calculated equilibrium temperatures of 58° to 69° C. The most important change is the transformation of smectite to illite within the mixed-layer I/S phase which occurs according to the reaction suggested by Boles and Franks (1979) with Al3+ acting as an immobile component. The source of K+ for this reaction is discrete illite. The breakdown of discrete illite results in two other changes with depth: the formation of kaolinite; and the increase of plagioclase feldspar which is due to reaction with Na+ and Ca2+ provided by the smectite to illite transformation. The Boles and Franks model is compatible with a steady supply of original mixed-layer I/S during the depositional history of the McAllen Ranch area. The constant content with depth of calcite, quartz, and potassium feldspar indicates that limited material, if any, is supplied by the shales to surrounding sands. The ions generated by changes within the clay minerals are involved in further clay mineral reactions as outlined above. In addition, magnesium and iron are involved in forming chlorite within the shales.

Clay Mineralogy and Depositional History of Frio Formation in Two Geopressured Wells, Brazoria County, Texas: ABSTRACT

Twenty-three shale samples, ranging in depth from 5,194 to 13,246 ft (1,583 to 4,037 m), from Gulf Oil Corp. 2 Texas State Lease 53034 well, and 33 shale samples, ranging in depth from 2,185 to 15,592 ft (666 to 4,752 m), from General Crude Oil Co./Department of Energy 1 Pleasant Bayou well were examined by X-ray techniques to determine the mineralogy of the geopressured zone in the Brazoria fairway. Both wells have similar weight-percent trends with depth for a portion of the mineralogy. Calcite decreases, whereas plagioclase, quartz, and total clay increase slightly. Within the clays, illite in mixed-layer illite-smectite increases and smectite in mixed-layer illite-smectite decreases. Four minerals have distinctly different trends with depth for each well. In the 2 Texas State Lease 53034 well, potassium feldspar and mixed-layer illite-smectite decrease, kaolinite increases, and discrete illite is constant. In the 1 Pleasant Bayou well, potassium feldspar and kaolinite are constant, mixed-layer illite-smectite increases, and discrete illite decreases. The most important diagenetic change in each well is the transformation of smectite to illite within the mixed-layer phase which occurs according to the reaction suggested by J. R. Boles and S. G. Franks with Al3+ acting as an immobile component. This change begins at calculated equilibrium temperatures of 89 to 92°C. The decrease in calcite and the lack of chlorite in the shales suggest that carbonate, iron, and magnesium migrate out of the shale in each well. In the 2 Texas State Lease 53034 well, the Boles and Franks reaction is consistent with a steady supply of original mixed-layer illite-smectite during deposition. Potassium feldspar provides K+ for the smectite to illite transformation. The breakdown of potassium feldspar also results in the formation of kaolinite and the increase of plagioclase feldspar, which is due to the reaction with Na+ and Ca2+, provided by the smectite to illite change. In the 1 Pleasant Bayou well, the Boles and Franks reaction is consistent with an unusually high mixed-layer illite-smectite content in the early depositional stages. The source of K+ for the smectite to illite reaction is discrete illite. The breakdown of discrete illite results in both the formation of kaolinite and the increase in plagioclase feldspar. End_of_Article - Last_Page 1430------------

Shale Mineralogy of General Crude Oil and Department of Energy 1 Pleasant Bayou Geopressured-Geothermal Test Well, Brazoria County, Texas: ABSTRACT

Thirty-three shale samples, ranging in depth from 2,185 to 15,592 ft (666 to 4,752 m), were examined by X-ray diffraction methods to determine changes in a mineralogy with depth. Quartz is present in all samples and averages 15 wt. %. Above 7,800 ft (2,377 m), calcite content varies due to fossil fragments. Below 7,800 ft (2,377 m), calcite content varies from 0 to 9 wt. %. Potassium feldspar and plagioclase contents are essentially constant at an average of 3 and 4 wt. %, respectively. Total clay content, combining kaolinite, illite, mixed-layer illite-montmorillonite (I/M), and traces of chlorite, is essentially constant, averaging 65 wt. %. Individual clay minerals have quite variable contents from sample to sample, but distinct trends are noted: (1) kaolinite content s constant at an average of 25% total clay; (2) illite content initially averaged 35% total clay, decreases with depth, and is zero in 10 of the 14 samples below 10,000 ft (3,048 m); and (3) mixed-layer I/M averages 40% total clay in shallow samples and 70% in deeper samples. The top of the geopressured zone, occurring at a pore fluid pressure gradient of 0.465 psi/ft and equilibrium temperature of approximately 190°F (88°C), is marked by a definite increase of illite in mixed-layer I/M at approximately 8,500 ft (2,591 m). A major change in illite content from 40% at 11,210 ft (3,417 m), to 84% at 11.750 ft (3,581 m), corresponds to a pore fluid pressure gradient of 0.7 psi/ft, and equilibrium temperature of approximately 250°F (122°C). In addition, the arrangement of the I/M layers changes from random interstratification in samples from 11,540 ft (3,517 m), and shallower, to an ordered layering from 11,750 ft (3,581 m) and deeper. End_of_Article - Last_Page 709------------

Clay Minerals as Indicators for Depositional Environment in South Halletsville Field, Lavaca County, Texas: ABSTRACT

The South Halletsville field, Lavaca County, Texas, has gas and condensate production from lower Wilcox sandstones and shales which have been interpreted as either channel turbidite deposits in outer-shelf to slope locations or deltaic and strand-plain sands and muds. Twenty-four core samples from the General Crude Oil Co. 1 A. G. Henkes Gas Unit were analyzed by X-ray diffraction methods to determine whether a semiquantitative estimate of clay mineral content would aid in determining the depositional environment. Discrete illite and chlorite are of particular interest because the presence of these minerals is interpreted as being due to original deposition. Three shale samples, from 10,194 to 10,206 ft (3,107 to 3,111 m) and 11 sandstone samples, 10,180 to 10,194 ft (3,103 to 3,107 m) were selected from one core section. This sequence of samples is particularly important because it contains a shale and the overlying sandstone. In addition, a deeper sandstone was sampled in the interval of 11,032 to 11,072 ft (3,363 to 3,375 m). If a turbidity-type event had occurred, the weight percent of non-diagenetic clays should (1) decrease significantly as the boundary is crossed between the shale and the overlying sandstone, and (2) gradually increase in progressively shallow samples within the sandstones. However, the weight percent for chlorite does not vary significantly regardless of a change in lithology, shale to sandstone, or a change in depth. The illite content gradually decreases with shallower depths in both core intervals. This sequence is more compatible with a transgressive deltaic environment. End_of_Article - Last_Page 1557------------