M. Elgmati, H. Zhang, M. Zobaa, B. Bai, and F. Oboh-Ikuenobe June 15 th, 2011
Purposes Palynofacies Analysis Kerogen Type Thermal Maturation Estimated Key Geochemical Parameters Total Organic Carbon 3D Submicron Pore and 2D Organic Matter Modeling Conclusions Acknowledgment 2
Conventional standalone analyses are inadequate and not suited for unconventional gas rock characterization Palynofacies analysis identifies intervals of exploratory interest in terms of hydrocarbon content The relatively inexpensive nature of palynofacies analysis makes it powerful in preliminary exploratory studies limited by tight budgets Pore imaging and modeling allows the evaluation of gas storage quantity and deliverability in shale-gas plays 3
The palynological study of depositional environments and hydrocarbon source rock potential based upon the total assemblage of particulate organic matter (Tyson, 1995) Palynofacies analysis was carried out on five samples Three from the Utica Shale (two samples from Dolgeville member and one sample from the Indian Castle Member) One sample from Haynesville shale One sample from Fayetteville shale 4
Conventional palynological processing technique: 1. Crush 10 15 grams of the sample in a mortar to the powder size 2. Treat the samples with concentrated HCl for 24 hours to remove their carbonate content 3. Dissolve the silicate fraction with HF treatment for 72 hours 4. Wash and sieve samples to remove clay particles and concentrated organic matter 5. Retain kerogen particles that range in size between 10 106 µm to make the final microscopic slides 6. Examine slides microscopically in transmitted light using variable magnification powers for analysis and photomicrography 7. Count 500 kerogen particles from each slide and classify them into four main categories namely, structured phytoclasts, degraded phytoclasts, opaques, and palynomorphs 5
Kerogen type IV was identified from all the studied samples, although they differ in the percentages of individual kerogen components Kerogen type IV was described (Peters and Cassa, 1994) as dead carbon, which has little or no hydrocarbon generating capability The examined samples (except sample #3 from Utica Shale) likely initially contained kerogen type III (gas prone material) that converted to type IV during the process of thermal over maturation 6
Haynesville Shale at 12,000 ft Palynomorphs 0% Opaques 36% Degraded phytoclasts 14.8% Structured phytoclasts 49.2% 1 Kerogen components identified from Haynesville Shale are dominantly phytoclasts and opaques Palynomorph-like particles were observed, but could not be confirmed due to their high degree of degradation and very dark color 7
Utica Shale, Indian Castle Mb. at 4,649 ft Palynomorphs 0% Opaques 22.6% Degraded phytoclasts 13.6% Structured phytoclasts 63.8% 2 Kerogen components identified are dominantly structured and degraded phytoclasts Palynomorphs (essentially chitinozoans) were very rare and very dark brown to black in color, many of them were broken down. 8
Utica Shale, Dolgeville mb. at 4,878 ft Palynomorphs 0% Structured phytoclasts 3.8% Degraded phytoclasts 0% Opaques 96.2% 3 An overwhelming abundance of black opaques with rare dark brown structured phytoclasts were found The majority of opaque particles were equant in shape and smaller in size than those recovered from other samples 9
Utica Shale, Dolgeville mb. at 5,197 ft Palynomorphs 0.2% Opaques 23.6% Structured phytoclasts 11.8% Degraded phytoclasts 64.4% 4 High abundance of very dark degraded phytoclasts along with black opaques and dark to very dark brown structured phytoclasts Palynomorphs were very rare Samples from the Dolgeville member of the Utica Shale at different depths were very different in their kerogen composition 10
Fayetteville Shale at 2,351 ft Palynomorphs 0% Structured phytoclasts 3% Degraded phytoclasts 3.8% Opaques 93.2% Very high abundance of black opaques in association with very little structured and degraded phytoclasts No palynomorphs were observed during the counting process 5 Almost all of the kerogen particles in this sample were equant in shape 11
Total Organic Carbon (TOC) Live Carbon Dead Carbon Gas Oil Organic Matter (Kerogen) Oil Prone Gas Prone Modified after Jarvie, 2004 Total Organic Carbon (TOC) Gas Oil Organic Matter Total Organic Carbon (TOC) Gas Oil OM Dead Carbon Dead Carbon Total Organic Carbon (TOC) Gas Oil OM Dead Carbon Total Organic Carbon (TOC) Dead Carbon Increased Maturation 12
1 2 3 4 The wall color in photomicrographs of the chitinozoan specimens identified from Utica and Haynesville shale samples ranges from dark brown to nearly black indicating postmature thermal phase Shale sample from Dolgeville member at the depth of 4,878ft has generated little dry gas, or nothing Other samples (with initial kerogen type III content) have generated wet gas and condensate All the samples currently contain thermally post-mature type IV kerogen, their source potential is limited to minor amounts of dry gas, or barren, at the present time 13
%Ro=0.55 %Ro=0.70 %Ro=0.90 %Ro=1.10 %Ro=1.40 Modified after Jarvie, 2004 Producible gas may be found at ~ 1.0% R o Adopted form Traverse, 2007 14
Qualitatively estimate some key organic geochemical parameters such as vitrinite reflectance (Ro %) and thermal alteration index (TAI) The dark to very dark brown colors of palynomorph walls in the studied samples(excluding Fayetteville Shale sample), which are typical post-mature source rocks, correspond to -4 to 4 TAI and 1.5 2.5% vitrinite reflectance (Traverse, 2007). This further suggests these source rocks are mainly in the metagenesis thermal alteration stage indicative of about 150 200 C temperature range (Peters and Cassa, 1994). 15
Total Carbon (wt. %) 12 10 8 6 4 2 0 #1 #2 #3 #4 #5 Inorganic C 1.16 5.21 9.69 8.71 1.09 Organic C 0.81 1.31 0.31 1.12 4.04 Sample #1 : Haynesville Shale at 12,000 ft Sample #2 : Utica Shale, Indian Castle Mb. at 4,649 ft Sample #3 : Utica Shale, Dolgeville mb. at 4,878 ft Sample #4 : Utica Shale, Dolgeville mb. at 5,197 ft Sample #5 : Fayetteville Shale at 2,351 ft The studied samples were quantitatively investigated in the laboratory for TOC analysis The analyzed samples have TOC contents of 0.81 4.04 wt% It is likely that most of the TOC, at present, is dead carbon Inorganic carbon content was also observed in Utica Shale samples, which is likely resulted from high concentrations of calcite (CaCO3) in this shale-gas play (Elgmati et al., 2011) 16
Submicron pore imaging and modeling provide insights into the petrophysical properties of shale-gas source rocks such as pore size histogram, porosity, and TOC. A dual beam system (SEM/FIB) was utilized to reconstruct the 2D kerogen model and the 3D pore model of shale-gas plays. A successful example of reconstructed submicron pore model from Fayetteville shale-gas sample is presented. 200 2D SEM images were used to reconstruct the original 3D submicron-pore structure. 17
SEM image showing the organic matter Converted 2D binary image of 0 and 1 pixel values The extracted TOC value is 3.91% (vs. 4.04 wt.% in TOC test). 18
Spectrum 1 Spectrum 2 Spectrum 3 Spectrum 1 Spectrum 3 Elements Atomic% C 37 % O 63 % Elements Atomic% C 42 % O 58 % Elements Atomic% O 83 % Al 16 % Si 1 % Spectrum 2 Dark porous spots represent kerogen materials which contain high organic carbon contents The solid part is believed to represent aluminum silicate class mineral (possibly illite) 19
3D model after alignment and stacking Binary model of 0 and 1 voxel value Element boundaries determined 20
Major kerogen pore size is 30nm Few micron-sized pores exist in this 3D model 21
Kerogen type IV was identified from all the studied samples with different percentages of individual kerogen components. The observed palynomorphs implied high level of maturation. Measured TOC content ranged from 0.81 4.04 wt% in the studied samples. Pores of organic matters were found in nano size and occupied 40 50% of the kerogen body. Petrophysical properties of the original pore structures can be effectively extracted from reconstructed three-dimensional models. Good agreement between the computed TOC from the SEM image and the measured TOC in the laboratory. 22
Financial support for this work from the Research Parternership to Secure Energy for America (RPSEA) and the United States Department of Energy. American Chemistry Society Petroleum Research Funding Baker-Hughes Southwestern Energy 23
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