Low-temperature gas from marine shales: wet gas to dry gas over experimental time

Marine shales exhibit unusual behavior at low temperatures under anoxic gas flow. They generate catalytic gas 300° below thermal cracking temperatures, discontinuously in aperiodic episodes, and lose these properties on exposure to trace amounts of oxygen. Here we report a surprising reversal in hydrocarbon generation. Heavy hydrocarbons are formed before light hydrocarbons resulting in wet gas at the onset of generation grading to dryer gas over time. The effect is moderate under gas flow and substantial in closed reactions. In sequential closed reactions at 100°C, gas from a Cretaceous Mowry shale progresses from predominately heavy hydrocarbons (66% C5, 2% C1) to predominantly light hydrocarbons (56% C1, 8% C5), the opposite of that expected from desorption of preexisting hydrocarbons. Differences in catalyst substrate composition explain these dynamics. Gas flow should carry heavier hydrocarbons to catalytic sites, in contrast to static conditions where catalytic sites are limited to in-place hydrocarbons. In-place hydrocarbons and their products should become lighter with conversion thus generating lighter hydrocarbon over time, consistent with our experimental results. We recognize the similarities between low-temperature gas generation reported here and the natural progression of wet gas to dry gas over geologic time. There is now substantial evidence for natural catalytic activity in source rocks. Natural gas at thermodynamic equilibrium and the results reported here add to that evidence. Natural catalysis provides a plausible and unique explanation for the origin and evolution of gas in sedimentary basins.


Introduction
Marine shales release gas under isotherm gas flow at lowtemperatures [1]. Gas is released discontinuously, in distinct aperiodic episodes that continue over time. It is nonlinear kinetic behavior resembling chaotic catalysis by transition metals [2]. Trace levels of oxygen suppress gas emission, and gas compositions reflect equilibrium control. The recent disclosure of natural gas at thermodynamic equilibrium and catalytic gas from marine shales similarly constrained, strongly suggests natural catalysis as the source of natural gas [3].
The origin of natural gas remains controversial, however. Many believe that thermal cracking is the source and cite various pyrolysis simulation experiments to support this view [4][5][6][7][8][9][10][11][12][13][14]. Recent hydrous pyrolysis experiments would seem to rule out natural catalytic activity in general [15], and activity by transition metals [16] in particular. This conclusion was based on the premise that any natural catalytic activity that might exist would express itself under hydrous pyrolysis conditions. However, natural activity is a low-temperature phenomenon that is not observed at pyrolysis temperatures (> 300°C) [1].
Here we address catalytic gas generation under open and closed conditions to distinguish it from alternative explanations, desorption and thermal degradation in particular. Gas compositions change very little over time in thermal degradation experiments under open and closed conditions. Methane concentrations remain within a narrow range between 20 and 60% vol (C1-C5) irrespective of kerogen type, temperatures, or reaction conditions [17][18][19][20][21]. Desorption under isothermal gas flow follows firstorder kinetics with the lighter hydrocarbons eluting before the heavier hydrocarbons [22], as typically seen in gas chromatography. Both processes should give characteristic compositions over time, easily distinguishable from catalytic generation.
Rates of catalytic reactions are controlled by substrate concentrations at catalytic sites. They can vary in molecular weight if mass transport controls hydrocarbon transit to active centers. This is particularly the case in heterogeneous systems where active sites can be isolated from the hydrocarbon pools surrounding them. Mass transport can then control concentrations at active sites and thus product compositions. Under these circumstances, product compositions can be very different under gas flow where mass transport is active [22] and static conditions where it is not. In our systems, we would expect gas flow to carry higher hydrocarbons to active sites while static conditions will limit these sites to the hydrocarbons in-place. The purpose of this research was to see if there were dramatic differences in the gasses emitted under gas flow and static conditions consistent with catalytic generation as opposed to desorption or thermal degradation.

Results and Discussion
The kinetics of generation and desorption are different and their products reflect the differences. Desorption is first order and therefore yields characteristic exponential curves over time [22]. Rates are proportional to k a * [a], where k a is the first order rate constant and [a] is the concentration of hydrocarbon a in the rock's kerogen and bitumen. Under isothermal gas flow, the gas-phase ratio of two desorbing hydrocarbons, (a) and (b), will be proportional to k a * We analyzed gas compositions (% mol C 1 -C 5 ) from three shales under isothermal gas flow using a procedure described elsewhere [1]. Figure 1 shows the ratio (C 3 )/(n-C 4 ) increasing under He flow at 50°C (Floyd shale). A similar curve obtains from New Albany shale at 100°C, and a slightly declining curve is seen from Mowry shale at 50°C ( Table 1). The propane and butane released under isothermal gas flow is therefore generated under gas flow. The fact that it occurs at 50°C, is suppressed by oxygen, and is episodic [1], points to catalytic generation, as opposed to thermal generation.
Gas compositions (% vol C 1 -C 5 ) changed only moderately as Fig. 1 and Table 1  To replicate flow conditions without mass transport, samples were subjected to multiple closed reactions. Products were removed between reactions by syringe at ambient temperatures. This generated a series of products representing gas generation unaffected by gas-flow mass transport. The two procedures (gas flow and sequential closed reactions) should give similar products if mass transport makes no contribution to product compositions and substantial differences if mass transport controls product compositions. Seven sequential products from Floyd shale at 50°C are shown in Fig. 5 (Table 2) and six sequential products from Mowry shale at 100°C are in Fig. 6 ( Table 3).
The two shales emit entirely different gases under flow and closed conditions with two distinctions outstanding. First, overall compositions remained distinct throughout both procedures. The Floyd product was dominated by ethane under gas flow (Fig. 2) and propane and butane under closed conditions (Fig. 5). Mowry gas under flow conditions (Fig. 4) bears no resemblance to Mowry gas under closed conditions (Fig. 6). Secondly, gas compositions under closed conditions progressed to lighter hydrocarbons over time. The effect was subtle but clear in the Floyd experiment (Fig. 5), and dramatic in the Mowry experiment (Fig. 6). It is definitive evidence against des- Times are in minutes. Yields: Floyd, 57 μg C 1 -C 5 /g; New Albany, 1.2 mg C 1 -C 5 /g; Mowry, 25 μg C 1 -C 5 /g. % var is variance as % of mean. The dashes indicate no data. Average % variance is 14% in the Floyd data; 42% in the New Albany data; 42% in the Mowry data.
orption as the source of hydrocarbons in these experiments.
The stark differences between gas flow and closed conditions are also illustrated in data variance. Average variance (% of mean over time) in Floyd data was 14% under gas flow (Table 1) and 28% in closed reactors ( Table 2). The average variance in Mowry data went from 33% under gas flow (Table 1) to 740% under closed conditions (Table 3). It is noteworthy that the product in Fig. 4 bears no resem-blance to that in Fig. 6 for the same shale at the same temperature under flow and static conditions, respectively.
Mass transport accounts for these differences. Gas flow transports hydrocarbons at rates greatly exceeding diffusion rates [22,23]. We can therefore anticipate a uniform steady-state composition of hydrocarbons at catalytic sites reflecting equivalent rates of hydrocarbon delivery and The distribution of C 1 -C 5 hydrocarbons emerging from Floyd shale under helium flow at 50°C (Fig. 1) Figure 2 The distribution of C 1 -C 5 hydrocarbons emerging from Floyd shale under helium flow at 50°C (Fig. 1). C 4 and C 5 represent both isomers. 57 μg C 1 -C 5 /g were generated over the course of this experiment.
The distribution of C 1 -C 5 hydrocarbons emerging from New Albany shale under helium flow at 100°C Figure 3 The distribution of C 1 -C 5 hydrocarbons emerging from New Albany shale under helium flow at 100°C. C 4 and C 5 represent both isomers. 1.2 mg C 1 -C 5 /g was generated over the course of this experiment. New Albany shale (Dev./Miss.) is side wall core (1025 m) from a well in Union County, KY, Illinois Basin.
The distribution of C 1 -C 5 hydrocarbons emerging from Mowry shale under helium flow at 100°C Figure 4 The distribution of C 1 -C 5 hydrocarbons emerging from Mowry shale under helium flow at 100°C. C 4 and C 5 represent both isomers. 25 μg C 1 -C 5 /g was generated over the course of this experiment. Mowry shale (Cretaceous) is core (7700 m) from an unknown well in Colorado.  Yields in μmol C 1 -C 5 /g are cumulative. Gas compositions represent gas generated during that period, not cumulative gas composition. The dashed lines represent no data. Total gas yield was 24 μg C 1 -C 5 /g for the first reaction and 26 μg C 1 -C 5 /g for the duplicate reaction. % var is the variance in compositions over the indicated time periods as % of mean. The average % variance for the seven hydrocarbons is 28% for the first reaction and 24% for the duplicate reaction.
Concentrations (% mol) of C 1 -C 6 hydrocarbons generated in six sequential closed reactions, Floyd shale at 50°C (Table 2) Figure 5 Concentrations (% mol) of C 1 -C 6 hydrocarbons generated in six sequential closed reactions, Floyd shale at 50°C (Table 2). 'Reactions' refer to the six closed reactions in Table 2. Data points represent average values of two sequential reactions: 'Reactions 1 & 2' is the average value for 1 st hr and 2 nd hr; 3 & 4 is the average value for 3 rd and 4 th hr, and so on. C 4 -C 6 include all acyclic isomers.
Concentrations (% mol) of C 1 -C 6 hydrocarbons generated in five sequential closed reactions, Mowry shale at 100°C (Table 3) Figure 6 Concentrations (% mol) of C 1 -C 6 hydrocarbons generated in five sequential closed reactions, Mowry shale at 100°C (Table 3). 'Reactions' refer to the five closed reactions in Table 3. Data points represent average values of two sequential reactions: 'Reactions 1 & 2' is the average value for 1 st hr and 2 nd hr; 3 & 4 is the average values for 3 rd and 4 th hr, and Reaction 5 represents the last reaction (24 hr). C 4 -C 6 include all acyclic isomers.
The natural progression of wet gas to dry gas is rarely reflected in thermal cracking simulations. This is partly because ethane and propane will not crack at temperatures typically used in thermal cracking experiments [4][5][6][7][8][9][10][11][12][13][14][15]. Dry gas generation in the laboratory has been reported [27], but only at temperatures above 450°C where these hydrocarbons decompose. With half-lives in the tens of millions [25] to billions of years [26] at temperatures under 200°C, it is very unlikely that thermal cracking could be the source of dry gas in sedimentary basins.
Wet gas to dry gas is replicated here for the first time. It proceeds through natural catalytic activity carried from the subsurface. It is paleoactivity that has persisted over geologic time as opposed to artificial activity created under laboratory conditions. Shales express activity at ambient temperatures without chemical or thermal activation and lose activity only under artificial conditions, temperatures over 300°C or exposure to oxygen [1]. Marine shales and their counterparts should generate gas in the subsurface as they do in the laboratory, and therefore propose natural catalysis as the source of natural gas in sedimentary basins. It explains high methane concentrations in natural gas, methane enrichment over geologic time, and natural gas at thermodynamic equilibrium [3]. We reject the thermal cracking hypothesis because it cannot explain these observations.

Conclusion
Hydrocarbons released from Floyd, New Albany, and Mowry shales are products of a natural catalytic process under fluid dynamic control. Desorption of preexisting hydrocarbons makes no significant contribution to the products in our experiments and thermal degradation at these temperatures would be unprecedented and highly unlikely.
The sharp distinctions in yield and compositions between closed reactions and gas flow reactions reflect relatively uniform substrate compositions under gas flow and changing compositions in closed reactions.
These results add to the evidence [1,3] that the natural catalytic activity in carbonaceous sedimentary rocks is the source of natural gas.

Experimental
The Mississippian Floyd shale and Devonian/Mississippian New Albany shales are described elsewhere [1]. The Cretaceous Mowry shale is whole core (2500 m) from an unknown well in Colorado. Rock-Eval: S1 = 2.61; S2 = 9.33; S3 = 0.15; Tmax = 439. Total Organic Carbon (Leco) = 2.5. The experimental procedures used in sample preparation and product analysis are described elsewhere [1]. The experimental procedure under gas flow is also described in that publication. Closed experiments were carried out in 5 ml glass vials fitted with PTFE/SIL septa Yields in μmol C 1 -C 5 /g are cumulative. Gas compositions represent gas generated during that period, not cumulative gas compositions. Total gas yield was 43 μg C 1 -C 5 /g for the first reaction and 65 μg C 1 -C 5 /g for the duplicate reaction. % var is the variance in compositions as % of mean. The average % variance for the seven hydrocarbons is 743% for the first reaction and 628% for the duplicate reaction.
purchased from Cole-Parmer (Vernon Hills, IL, USA). Rocks were prepared for analysis by grinding to 60 mesh at ambient temperatures in glove bags under argon, placed in vials under argon, and sealed with open caps fitted with septa. The caps were then wrapped tightly with plastic electrical tape at the vial-screw cap interface to prevent leakage under heating. The vials proved to be leakproof for the duration of our experiments at 100°C. In sequential experiments, a charged vial containing about 1 gm sample was heated for a time segment, usually 1 hour at constant temperature (± 5°C), then cooled. Product gas was removed by syringe and analyzed as previously described. The vial was then purged of all hydrocarbons by flowing argon in and out of the vial through two needles inserted through the septa (15 min, ~5 ml/min). The purged vials were then reheated for a period of time, usually one hour, and purged again. This procedure was repeated several times. Hydrocarbon compositions in Tables 2 &3 reflect the gas generated in the time segments indicated. The Floyd shale was heated four times at 50°C for one hour, a fifth time for 19 hours at 50°C, and a final time for 19 hours at 50°C. The experiments were duplicated with another sample of Floyd shale (Duplicate Reaction, Table 2). The product from the 3 rd heating was inadvertently lost and therefore not analyzed. Duplicate reactions of Mowry shale were carried out in four sequential one-hour reactions at 100°C and a fifth reaction at 100°C for 24 hours (Table 3). Duplicate experiments did not use aliquots of 60 mesh shale. Different samples from the same source were subjected to the same experimental procedures: grinding in argon, sieving, and so forth. The variations in yield and product compositions shown in Tables 2 &3 therefore reflect heterogeneity in samples as well as the variance in our analytical procedure.