Kinetics of the opal-a to opal-ct phase transition in low- and high-toc siliceous shale source rocks

Transcription

1 Kinetics of the opal-a to opal-ct phase transition in low- and high-toc siliceous shale source rocks Danica Dralus Stanford University, now at BP Mike Lewan USGS, Denver, CO (retired) Ken Peters Schlumberger, Stanford University BPSM Annual Meeting November 12, 2014 Stanford, California

2 Outline Introduce opal-ct/quartz transition in the oil patch Recap opal-ct to quartz experiments* Describe new opal-a to opal-ct experiments Report preliminary opal-a to opal-ct kinetics data *For additional details, see my dissertation (Stanford, Aug 2013): Chemical interactions between silicates and their pore fluids: How they affect rock physics properties from atomic to reservoir scales 2

3 Siliceous deposits undergo phase transitions. post-doc work Opal-A is amorphous and easy to dissolve. grad student work Opal-CT is microcrystalline. Quartz is fully crystalline and hard to dissolve. They are all just SiO 2 in different arrangements! 3

4 Rock properties change with the phase transition, sometimes resulting in diagenetic traps. mineral density increases porosity increases permeability increases brittleness increases for quartz This is a known trapping feature in the San Joaquin Basin. 4

5 Finding diagenetic traps with seismic data is tricky. Resolution Interpretation interpretation 1: fluid interface interpretation 2: phase change Chico Martinez Creek; Courtesy of R. Behl, CSULB 5

6 We need independent estimates of the opal-ct and quartz transition depths. Goal: predict the transition depths and their timing in the petroleum system evolution Tool: predictive geochemistry hydrous pyrolysis experiments (kinetics) kinetics for opal-ct to quartz transition kinetics for opal-a to opal-ct transition grad student work kinetics for high-toc opal-a to opal-ct transition post-doc work 6

7 We want kinetics that reflect subsurface conditions. natural samples (w/ natural contaminants) plenty of fluid to circulate buffered aqueous solution to maintain high fluid ph (ph after pyrolysis) assemble cook recover 7

8 Reaction rate is determined using XRD spectra and kinetics models. For the best fit line in the Arrhenius plot, the intercept gives you A 0 and the slope gives you E a. Each sample s reaction progress is determined using XRD. fraction of rx complete The slope of the best fit line at each temperature is the reaction rate. 8

9 Our earlier work examined the opal-ct to quartz transition. Monterey Fm. porcelanite (opal-ct, carbonate) from Lompoc, CA quarry no TOC, no clays data fit well by nucleation and growth kinetics approximately equivalent to using zero-order kinetics A 0 = 1.96 x 10 9 hr -1 E a = kcal/mol 9

10 Recent experiments focused on the opal-a to opal-ct transition. Monterey Fm. samples from the same Lompoc quarry weathered and unweathered samples with varying amounts of TOC 10

11 Samples showed large variability. TOC (wt% HC) HI (S2x100/TOC) Tmax ( C) WEST EAST massive claystone both ~10% phyllosilicates laminated claystone TOC (wt% HC) HI (S2x100/TOC) Tmax ( C)

12 There are many challenges to conducting these experiments, especially off campus. Limited time to conduct experiments (6 weeks) Nearly 80 individual experiments No way to know a priori how long to cook samples Proper analysis requires detailed XRD which can t be done on the fly. Determination of silica phases takes a skilled analyst. 12

13 Preliminary results 13

14 Reminder: These data can be used in PetroMod. opal-a to opal-ct transition opal-ct to quartz transition opal-ct quartz no transition 14

15 Summary We have opal-a to opal-ct (to quartz) transition data for high- and low-toc samples. Low-TOC samples undergo transformation much faster at experimental temperatures than high-toc samples. There may be enough data for determination of kinetics parameters. We can at least put bounds on the transition rates. Kinetics will be an improvement over the current modeling techniques (nomogram or dissolution rates). 15

16 Acknowledgements 16