Evaporite-associated source rocks

A hydrocarbon source rock s generally considered to be a fine-grained rock that, during its burial and heating, generates and releases enough fluids to form commercial accumulations of oil or gas (Figure A). Back in 1981, Kirkland and Evans made the observation that some 50% of the world’s oil sequestered in carbonate reservoirs may be associated with mesohaline micritic source rocks. Heresy or not, the notion that much of the oil in carbonate reservoirs, sealed by evaporite salts, may have been sourced in earlier less saline, but still related, evaporitic (mesohaline) conditions, is worthy of consideration (See Warren 2016, Chapter 10 for details and the literature base).

The association between mesohaline waters, the accumulation of organic-rich sediments and the evolution of the resulting evaporitic carbonates into source rocks has been noted by many, including: Woolnough, 1937; Sloss, 1953; Moody, 1959; Dembicki et al., 1976; Oehler et al., 1979; Malek-Aslani, 1980; Kirkland and Evans, 1981; Jones, 1984; Hite et al., 1984; Eugster, 1985; Sonnenfeld, 1985; Ten Haven et al., 1985; Warren, 1986; Evans and Kirkland, 1988; Busson, 1991; Edgell, 1991; Beydoun, 1993; Benali et al., 1995; Billo, 1996; Aizenshtat et al., 1998; Carroll, 1998; Schreiber et al., 2001; Love et al., 2007;Schnyder et al., 2009; Warren 2011; Comer, 2012.

As long ago as the middle of last century, Weeks (1958, 1961) emphasised the importance of evaporites as caprocks to significant hydrocarbon accumulations. He also pointed out that many of the cycles of deposition that involve organic-rich carbonate marls or muds also end with evaporites. Many of these authors noted an association with Type I-II hydrogen-prone kerogens in evaporitic source rocks and related this to the ability of halotolerant photosynthetic algae and cyanobacteria to flourish in mesohaline waters. Such kerogens tend to be oil-prone rather than gas prone and typified by long-chain hydrocarbons (Figure B).

Organic matter buried in organic-rich sediment undergoes numerous compositional changes before it becomes a source rock, at first dictated by microbial agencies and then by thermal stress (Horsfield and Rullkotter, 1994). This continuum of processes is called thermal maturation. Based on vitrinite reflectance (Ro) measurements of macerals, a maturing source rock can be divided into three consecutive stages: eogenesis (diagenesis) (Ro<0.5%); catagenesis (0.5%<Ro<2.0%); and metagenesis (2.0%<Ro<4.0%), with the oil generation window typically in the Ro range 0.5-1.3% (≈65-150°C).

Around 90 percent of the organic material in a source rock at the onset of catagenesis is dispersed kerogen, consisting of a range of residual materials with basic molecular structures that are dominated by stacked sheets of aromatic hydrocarbon rings, in which atoms of sulphur, oxygen, and nitrogen also occur. Attached to the ends of the rings are various hydrocarbon compounds, including normal paraffin chains. Mild heating of kerogen over long periods results in the cracking of the kerogen molecules and the release of the attached paraffin chains. Further heating, perhaps assisted by the catalytic effect of clay mineral transformation and evaporite dewatering in the source rock matrix, may then produce soluble bitumen compounds, followed by the various saturated and unsaturated hydrocarbons, asphaltenes, and others of the thousands of hydrocarbon compounds that make up crude oil mixtures. Thus kerogen cracking is induced by burial and heating, and it releases micropetroleum (a mix of liquid and gaseous hydrocarbon products that are soluble in organic solvents) into the pore system of the source rock.

Much of the organic matter preserved in evaporitic carbonates, and the resulting source rocks, originated as planktonic blooms (pelagic “rain from heaven”) or from the benthic biomass (“in situ” accumulations). Such organics typically settled out as seriate pulses of organic matter (often pelleted) that sank to the bottom of a layered brine column. Each pulse was tied to a short period when surface brines were diluted and halotolerant producers (mostly cyanobacteria and algae) flourished in the freshened lit zone. That is to say, the laminated mesohaline mudstones that constitute evaporitic source rocks reflect biological responses to conditions of “feast or famine” in variably layered bodies of brine. Warren (1986, 2011, 2016) referred to the schizohaline salinity cycles of saline settings as repeated episodes of “life or death.”

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Organics and source rocks. A) Relative volumes of gases and liquids derived from a sapropelic and a humic source rock (after Hunt, 1996). Evaporitic source rocks tend to be sapropelic and contain less iron as a reductant so they are also likely to generate more H2S. B) Molecular structure of typical oil-generating (sapropelic) and gas-generating (humic) organic matter showing the greater preponderance of long-chain hydrocarbons in the sapropelic material (after Hunt, 1996).
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Kerogen (after Cornford, 2005) A) Simplified classification of organic matter in sedimentary rock. Bitumen is extracted from ground rock using organic solvents and consists of hydrocarbons and non-hydrocarbons. Treatment of the extracted residue with hydrochloric (HCl) and hydrofluoric (HF) acids removes most carbonate and silicate minerals, respectively and leaves behind the kerogen. This kerogen residue consists of various macerals that can be identified by organic petrography. B) Approximate equivalence of terms used to describe kerogen using different techniques for its study.

Reflectivity is measured by shining a beam of monochromatic light (with a wavelength of 546 nanometres) on to a polished surface of the vitrinite macerals and measuring the percentage of the light reflected with a photometer. Put simply, it measures the degree of darkening (roasting?) of the organics associated with the heating of the rock during burial and is not a direct indication of oil maturation. Macerals are recognizable remains of different types of organic matter that can be differentiated under the microscope by their morphologies. As originally defined, macerals are the microscopic organic components of coal and consist of an irregular mixture of different chemical compounds, which are analogous to minerals in inorganic rocks, but differ from minerals in that they have no fixed chemical composition and lack a definite crystalline structure (Figure). Macerals change progressively both chemically and physically during ongoing burial and heating; as the rank of macerals (coaly material) advances or the maturity of the organics increases.

Macerals in any sediment can be classified into four major groups: vitrinite, inertinite, alginite and exinite (Figure A). Liptinites broadly encompass the alginites and exinites. Vitrinite is derived mostly from woody plant tissue and includes the macerals collinite and telinite; most coals have a high percentage of vitrinite macerals. (Figure B) The inertinite group comprises fusinite, micrinite, sclerotinite, and semi-fusinite, which are all rich in carbon but poor sources of volatiles. The exinite/alginite (liptinite) macerals, characterized by high hydrogen content (and hence excellent liquid hydrocarbon-generating potential), include alginite, cutinite, resinite, and sporinite (Figure 9.2b). Alginites are commonplace macerals in evaporitic settings, while vitrinites are rare, reflecting a lack of higher plants in most hypersaline settings. A lack of vitrinite and poor calibrations to alginite maturation indices can lead to weak correlations between actual source rock maturity and that indicated by vitrinite-based estimates in evaporitic source rocks. This is the case for the Type II source rocks in the Hanifa-Hadriya interval in Saudi Arabia (Ayres et al., 1982), which contains little or no vitrinite.

Reflectance measurements determined from a few phytoclasts in such mesohaline source rocks may be unreliable and other geochemical evidence, such as thermal alteration indices (TAI) or biomarker maturity parameters, should always support Ro measurements of maturity in such sediments. Significant proportions of oil-prone macerals or bitumens, as are common in many mesohaline source rocks, tend to retard the normal progression of vitrinite reflectance with maturity (Price and Barker, 1985). Transformation of organic matter to liquid and gaseous hydrocarbons is the result of thermal cracking of kerogen. Kerogen is the principal global precursor of petroleum and its formation, mostly via biologically-induced alteration of plant and animal detritus, its formation is complete by the end of eogenesis. 

The maturity and evolution of the kerogen are paralleled and indicated by increasing maceral (vitrinite) maturity (Table). Once released into the pores of the source rock, micropetroleum migrates toward the surface via secondary migration pathways, and along the way, a portion of this material can be trapped in a variety of reservoirs of varying quality. The nature of the organic precursors to kerogen exerts a strong influence on kerogen structure and bulk composition, and hence on subsequent oil- and gas-generating characteristics. Source rock kerogens enriched in long-chain hydrogens (aliphatics or paraffins) tend to be oil-prone type I sapropelic source rocks and are associated with algal, bacterial and archaeal precursors (indicated by C2H6+ in Figure). Kerogens that are strongly depleted in long-chain hydrogens are typically type III humic (vitrinitic) source rocks, associated with organics sourced from higher plants (Figure). They are gas prone, and the gaseous products are dominated by methane (dry gas), sometimes with associated hydrogen sulphide or nitrogen. Like hydrocarbons, hydrogen sulphide is generated both biochemically and thermochemically.

Not all source rocks are evaporitic. Organic matter accumulating in mesohaline sediment is derived from the activities of the halotolerant and halophilic biomass, mostly algae, bacteria and archaea. Kerogen derived from the burial of this organic matter will be composed of their somewhat altered remains, as will the expelled hydrocarbons. Sapropelic organic matter is created by decomposition and polymerisation of high-lipid organic materials (with aliphatic long-chain hydrocarbons), which are commonplace in algal and bacterial sediments deposited under anaerobic conditions. Hence, sapropelic organic matter tends to be associated with oil-prone Type I or Type II proto-kerogens. Humic organic matter tends to contain less long-chain hydrocarbons, and so constitutes a large proportion of the more gas-prone source rocks found in coaly sediments and dispersed terrestrial organics deposited in prodelta settings. It is mostly associated with Type III protokerogens. Unoxidised evaporitic organic matter tends to be hydrogen-rich and oil-prone; its sapropelic kerogens (Type I or Type I-II) contain higher proportions of long-chain hydrocarbons from cyanobacterial, archaeal and bacterial precursors. Higher plant material, which is the source of much of the humic component in nonevaporitic kerogen simply does not accumulate in large volumes in an evaporitic source rock; higher plants need fresh water and do not flourish in hypersaline waters.

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The generation defined oil and gas window approximately related to some common maturity parameters. Figures above also shows relationship between maturation, Ro and temperature (after Cornford, 2005).
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Worldwide distribution of evaporite-related source rocks named with respect to hosting basins (replotted from SaltWork GIS database version 1.9)

Mesohaline source rock settings

A common theme to most source rocks, both evaporitic and nonevaporitic, is that they require anoxic conditions to accumulate and preserve their organics. This, in turn, requires some way of restricting the influx of oxygenated waters to the depositional site and is usually accomplished by thermal or density stratification of a water body, typically in a setting where plankton flourish periodically in the upper water mass. This is why so many subaqueous evaporitic settings are also settings where organics tend to accumulate.

By definition, evaporites require restricted inflow conditions to accumulate, and they are also typically areas of density-stratified waters with high salinities in the lower denser more saline water mass. Levels of dissolved gases are negligible to non-existent in the lower brine body as it does not easily exchange with the atmosphere, so encouraging long-term bottom anoxia. This does not mean all of the world’s source rocks were deposited in evaporitic settings, only that the restricted conditions that favour the accumulation of evaporites are similar to conditions that support the accumulation of some types of organic-rich bottom sediments, especially in the interval that precedes deposition of actual salts. In arid climates, any minor changes in inflow conditions, such as hydrographic isolation from the ocean, can quickly force a transition from one into the other. Hence the common association of source rocks with layered basin waters typical of the early mesohaline phase and the onset of evaporitic sedimentation in many lacustrine/epeiric settings

In the marine realm, the same need for tectonic or eustatic restriction to create a basin suitable for source rock accumulation (intrashelf, circular or linear sag) also explains why, if further restriction and hydrographic isolation from the ocean ensues, the same region is easily covered by a sequence of salts. Worldwide, studies of ancient evaporitic basins have shown that organic-rich mesohaline sediments can accumulate beneath ephemeral surface brines, in salterns, or in basin and slope settings in both marine and continental settings (Kirkland and Evans, 1981; Oehler, 1984; Warren, 1986; Evans and Kirkland, 1988; Rouchy 1988; Busson, 1992). The most prolific accumulations of organics in ancient evaporitic sediments tend to have been deposited as laminated micritic carbonates beneath density-stratified moderately-saline (mesohaline) anoxic water columns of varying brine depth.

There are three, possibly four, primary mesohaline density-stratified settings where organic-rich laminites (source rocks) accumulate in saline settings that are also associated with, or evolve into, evaporite deposits (Figure): 1) Basin-centre lows in marine-fed evaporitic drawdown basins (basinwide salts).2) Mesohaline intrashelf lows atop epeiric evaporitic platforms.3) Saline-bottomed lows in perennial underfilled saline lacustrine basins.4) Closed seafloor depressions in halokinetic deepwater marine slope and rise terrains. For a detailed listing and discussion of examples in all four groups, see Warren 2016, Chapter 9. 

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Dominant depositional styles for evaporitic source rocks (shaded green). All the water bodies show evidence of salinity-related density stratification (indicated by dashed line), this is also reflected in an associated heliothermic stratification. (DHAL = Deepsea hypersaline anoxic lake) (from Warren, 2016).
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