Thursday, October 2, 2014

How Are Diagenetic Studies Useful In Understanding Sedimentary Basin History

I dusted of my PhD dissertation last week for two reasons. A friend insisted that she wanted to see my research.. and then this paper in the Journal of Sedimentary Research (behind paywall):

Diagenetic Evolution of Selected Parasequences Across A Carbonate Platform: Late Paleozoic, Tengiz Reservoir, Kazakhstan by J. A. D. Dickson and J. A. M. Kenter

The work is eerily similar to what I did for my PhD which was carrying out a detailed study of cementation patterns in Middle and Late Ordovician carbonate parasequences from the southern Appalachians.

Dickson and Kenter use petrographic techniques along with cathodoluminescence to tease apart the cementation sequence and pore space modification of the carbonate rocks. Hydrocarbon reservoir quality depends in part on how reaction of sediment with water either dissolves material to create pore space or precipitates cements to modify pore space. So, understanding the timing of these events in the context of the burial history of the sediment pile on a basin wide scale can help geologists predict reservoir quality.

Ok, so what are Parasequences?

Parasequence is a layer of sediment representing deposition over a few tens of thousands of years. Deposition is episodic since sea level may rise and fall over a frequency of tens of thousands of years and so layer after layer of sediment (parasequences) may be stacked to form a thick sedimentary basin fill. Parasequence deposition may be halted by a sea level fall. The sea floor may become land, exposed to rain water and groundwater circulation. Fresh water reacting with the carbonate sediment will dissolve skeletal material creating pore space but also deposit cement (a chemical precipitate), thus filling pore space. Early cements often make sediment rigid, preventing it from being compacted upon burial. This actually preserves porosity even when the sediment is deeply buried.

This sort of a punctuated parasequence deposition may continue for tens of millions of years. This means that there will be scores to hundreds of discrete episodes of parasequence deposition and diagenetic alteration. But the pattern of alteration affecting each parasequence may not be identical because the extent of sea level fall and the amount of fresh water penetration into the sediment may change over time. Thus, parasequences or bundles of parasequences often have unique early diagenetic histories that may create either patchy or pervasive porosity networks that control fluid flow including migration of oil later in the history of the basin. Geologists want to understand early diagenesis and quantify this pore space evolution for predicting reservoir quality.

That essentially was the problem I studied in the Ordovician sequences of the southern Appalachians. Middle Ordovician deposition took place in a greenhouse climate with moderate sea level falls and limited fresh water diagenesis. On the other hand Late Ordovician deposition took place in an icehouse regime (Richmondian-Hirnantian glaciation) with large sea level falls (because sea water gets locked up in polar ice caps)  and extensive exposure related (vadose zone) diagenesis.

Ok, so this is a broad overview, but what techniques do geologists use to study cements?

Cements are chemical or biochemical precipitates. Their composition, texture and fabric reflect ambient conditions of temperature, pore fluid chemistry and the mineralogy of the host carbonate.

The first step is to use conventional petrographic studies to recognize different types of cements and to establish the sequence of diagenetic or cementation events. This is done using pretty much the same principles used in lithostratigraphic mapping i.e. order of superposition, unconformities and cross-cutting relations. The oldest cement generation is the one immediately overlying a host substrate, which could be a skeletal grain or a detrital clast. A younger cement generation will overlie or cut across the older cement. A cement stratigraphy can thus be established.

Cement generations will also differ chemically and these differences can be mapped by their different response to synthetic staining agents. For example ferroan calcite stains blue in  potassium ferricyanide. Calcite stains pink in Alizarin Red-S and is distinguised from dolomite which does not take that stain.

Cement generations can also be recognized using cathodoluminesence studies. Different generations of calcite may contain varying amounts of trace elements such as Mn and Fe. The intensity of luminescence in calcite cements has been found to be related to the concentrations of Mn+2 and Fe+2, which act as activator and quencher of luminescence  respectively. Reduction of Mn and Fe to a divalent state is necessary for these elements to enter the calcite lattice. In oxidizing pore-fluids, neither Mn+4 or Fe+3 is incorporated into growing calcite crystals, and thus cements are black (non-luminescent). In pore fluids with progressively lower Eh , reduction of Mn first and then Fe leads to their incorporation into the growing cements, giving the crystals a bright to dull luminescence. Image to the left shows this sequence.

However the overall context and stratigraphic relations are the most important since the same cement generation can develop chemically distinct characteristics across a basin or in different facies.

Cements also different in their stable isotopes of oxygen and carbon and trace elements signatures. These are used to further pinpoint the chemistry and origin of pore fluids.

Great, so then how does all this help geologists understand basin history?

Diagenesis is a complex process, and diagenetic products such as cements commonly contain a detailed record of fluid-circulation events affecting the basin at various stages in its burial history. When sampled on a regional scale, textural and chemical patterns in cements often indicate fluid source areas and directions of fluid flow.  A wealth of information regarding ancient fluid chemistry, its subsequent chemical evolution during cementation events, and the controls of facies and stratigraphic architecture on groundwater flow in basins can be obtained from documenting and interpreting cement patterns in carbonates.

My research revealed that both  the Middle Ordovician  and the Upper Ordovician sequences were affected by meteoric diagenesis relatively early in their individual burial histories. Detailed examination of the diagenetic products using petrography (light and cathodoluminescence ) and geochemistry (trace and stable isotopes) showed that the precipitation environments were very different.  Late Ordovician carbonates of southern Tennessee and northeast Georgia were exposed to subaerial diagenesis during Richmondian sea-level falls associated with the formation of the Taconic unconformity, and show substantial vadose (above the water table) and phreatic zone (below the water table) alteration. Image to the left show a faceted calcite crystal deposited in the vadose zone. It is a micro-stalactite a couple of millimeters or so in length, the roof here is an echinoid shell and the crystal is growing in the open pore space underneath. In the vadose zone, pores are not completely water saturated. Instead, water occurs as drops held by surface tension between and underneath grains. Cements precipitate from these drops of calcium carbonate saturated water.

The underlying Middle Ordovician limestones also contain abundant phreatic calcite cement, but cementation is not related to synsedimentary emergence, or to direct vertical infiltration of meteoric water sourced from the overlying Upper Ordovician unconformities. Instead, recharging meteoric water in basin-margin highlands to the southeast, entered Middle Ordovician limestones through confined aquifers during Late Ordovician to Early Silurian times. This meteoric cementation history suggests that patterns of groundwater flow in the basin were strongly influenced by regional shale and lime-mud facies that  occur between these two units. These low-permeability strata compartmentalized the Mid-Late Ordovician basin fill into contemporaneous, but hydrologically isolated surficial and deeper aquifers. These groundwater circulation systems lasted a few million years resulting in pervasive cementation and pore space destruction early in the history of the deposit. This means both the Middle and Late Ordovician parasequences had little porosity available at the oil generation and migration depths making them poor reservoirs.

Image below show a phreatic zone cement in cathodoluminescence filling the inside of a bivalve shell. Phreatic zone cements form below the water table in pore spaces completely filled with water, and so they line all the sides of the open space and grow inwards.

The significance of my work is that it showed the overriding influence of the tectonic setting and resulting stratigraphic architecture on imposing distinctive diagenetic signatures on sedimentary sequences deposited during changing climatic regimes.

Phew.. I know that's a lot to digest, but I find diagenesis a fascinating topic of study. Calcite crystals a few millimeters across hold secrets about sea level falls, glaciations, aquifer formation and groundwater flow. From the minute to the grand.. making these connections is what makes geology such an interesting field!

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