Diatomite

Key Takeaways

• Diatomite porosity is exceptionally high (40 to 80 percent total porosity) because the silica frustule structures contain both inter-particle pores (between individual diatom tests) and intra-particle pores (within the intricate holes and channels of individual frustule structures), resulting in a complex, multimodal pore system with pore throat diameters ranging from 1 to 100 micrometers for the inter-particle pores and less than 0.1 to 5 micrometers for the intra-particle pores; despite this high total porosity, the permeability of diatomite petroleum reservoirs is typically 0.001 to 0.1 millidarcy (tight, technically challenging to produce), because the small, tortuous intra-particle pore throats restrict fluid flow far more than the total pore space would suggest; this combination of very high porosity and very low permeability makes diatomite one of the most extreme examples of poor pore-throat connectivity in petroleum geology, analogous to (but geologically distinct from) tight shale reservoirs, and requires that fluid must be mobilized by thermal means (steam, SAGD, CSS) or by fracture networks created by hydraulic fracturing to flow at rates sufficient for economic production.
• California diatomite petroleum reservoirs of the San Joaquin Valley (Belridge, Cymric, McKittrick, Lost Hills) are among the most intensively developed unconventional oil accumulations in North America, with total estimated in-place resources of several billion barrels, of which recovery factors of 15 to 40 percent have been achieved through steam injection and hydraulic fracturing programs; the Monterey Formation diatomite in these fields was deposited during the Miocene in an upwelling marine setting that simultaneously produced abundant diatom blooms (which contribute the silica frustule matrix), organic material (which thermally matures into the oil that impregnates the diatomite pore system), and reducing bottom-water conditions (which prevent organic oxidation and preserve the kerogen); the diatomite of the Monterey Formation has been both source rock and reservoir for oil that migrated short distances from its point of generation, with the low thermal maturity of much of the diatomite interval (requiring steam heating to reduce oil viscosity) reflecting the relatively shallow burial and modest geothermal gradient of the San Joaquin basin.
• Diagenesis of diatomite through progressive burial and temperature increase drives a series of mineralogical transformations that significantly alter the reservoir quality: fresh diatomite consisting of opal-A (amorphous hydrated silica) at shallow burial depths progressively transforms to opal-CT (a poorly crystalline cristobalite-tridymite phase) at temperatures above approximately 50 to 80 degrees Celsius, and further to micro-crystalline quartz (chert) at temperatures above approximately 90 to 120 degrees Celsius; the opal-A to opal-CT transition reduces porosity by 15 to 20 percent and increases compressive strength (the rock becomes harder and more brittle), while the opal-CT to quartz transition further reduces porosity by an additional 10 to 15 percent and produces a dense, low-porosity chert that is no longer a petroleum reservoir regardless of the original depositional richness; the depth of these transitions varies with geothermal gradient, so that in areas with high heat flow (volcanic arcs, rift zones) the diagenetic transitions occur at shallower depths than in cool cratonic settings, with profound implications for the lateral extent of productive diatomite reservoirs within individual basins.
• Hydraulic fracturing in diatomite petroleum reservoirs is uniquely challenging because the very low compressive strength of uncemented diatomite (1 to 5 MPa unconfined compressive strength, compared to 20 to 60 MPa for typical sandstones and 50 to 200 MPa for carbonates) and its high compressibility cause the formation to compact and close fractures rapidly when the fracturing pressure is released, limiting the effective propped fracture half-length and conductivity; diatomite fracturing programs use low-viscosity fracturing fluids (water fracs with minimal gel) to minimize filter cake damage in the ultra-low-permeability matrix, very fine proppant (70/140 mesh or 100 mesh sand or ceramic) to suspend in the low-viscosity fluid and pack the fractures at the low closure stresses characteristic of shallow diatomite reservoirs (typically 6 to 15 MPa closure stress at 300 to 600 meter depth), and large fracture volumes relative to the rock volume contacted to create complex fracture networks in the naturally fractured diatomite; production from diatomite hydraulic fracture treatments is sensitive to the fracture conductivity and the thermal stimulation (steam flooding through the fracture network), and production rates decline rapidly after initial stimulation, requiring frequent refracturing or restimulation programs to maintain commercial rates.
• Industrial diatomite (diatomaceous earth, DE) applications consume approximately 2.2 million metric tons per year globally, with the United States (primarily from Lompoc, California, and Clark County, Nevada deposits), China, and Denmark being the largest producers; industrial DE is processed by crushing, drying, and calcining (heating to 900 to 1,050 degrees Celsius) or flux-calcining (calcining with fluxing agents such as sodium carbonate) to modify the particle structure and filtration characteristics, with the calcining process converting the white opal-A to pink or red cristobalite-rich material; industrial DE is used as a filter aid in beverage clarification (beer, wine, fruit juice), edible oil filtration, pharmaceutical manufacturing, and swimming pool filtration (where its high surface area and complex pore structure trap suspended particles); as a functional filler in paints, rubber, and plastics; as an absorbent for spills and hazardous materials (its natural capillary absorption capacity up to 150 percent of its own weight in liquid makes it effective for industrial spill cleanup); and as a mild abrasive in polishing compounds and toothpaste; the petroleum reservoir and industrial mineral applications of diatomite represent entirely separate markets with distinct quality specifications and pricing.