Diagenetic Traps

In certain instances, the reservoirs themselves, having already lost their reservoir properties because of diagenetic and catagenetic processes, may react as a barrier. Such barriers are tied to the unconformity but may be also of other origins such as  

The identification of such dia- or catagenetic barriers is of great importance in the search for traps as the accumulation of hydrocarbons may have taken place below a more pronounced barrier located above the unconformity. 

This type of dia- or catagenetic trap is developed in the north of the Oued el-Mya Basin in the regions of Haniet el-Beida (HEB),Kef el-Argoub (KG) and Hassi Ladjouad (HLJ), as illustrated in Fig. 2.14. 

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Unassociated with unconformities (channels, bars, and reefs) Diagenetic traps (due to solution or cementation)... 

The feasibility of mineral trapping of C02 in dawsonite is demonstrated by the Bowen-Gunnedah-Sydney Basin in Australia, which has abundant diagenetic dawsonite that formed in response to magmatic C02 (Baker et al. 1995). In addition, abundant dawsonite in the... 

Figure 5.7 Cross-sections showing configurations for diagenetic stratigraphic traps caused by cementation (A), solution (B) and shallow oil degradation (C) (from Elements of Petroleum Geology, by Robert C. Selley. Copyright ( ) 1985 by W.H. Freeman and Company. Reprinted by permission).

If it is assumed that fluid inclusions in ankerite are primary, measured Th values would represent minimum trapping temperatures (the presence of dissolved CH4 or hydrocarbons in these inclusions could not be ascertained). However, as ankerite pre-dates calcite and quartz in the diagenetic sequence, the lowest Th yielded by fluid inclusions in calcite and quartz, i.e. 85-90°C, may represent a maximum formation temperature for ankerite. This implies that diagenetic ankerite in the Oseberg must have formed at temperatures between 65 and 85° C, i.e. essentially identical to measured Th values (Table 2). 

Such dynamic overpressure systems (sic.) with traps experiencing cyclic fill-drain periods was proposed by Leonard (1993) and invokes intermittent breaching of the seal and subsequent annealing due to petroleum column loss, progressive burial and diagenetic cementation. 

Fig. 20. Fluid inclusions in diagenetic minerals like quartz may represent hermetically sealed testers of the fluid existing in fields and dry structures at the time of inclusion formation. Chemical analysis of hydrocarbon gas and oil in such inclusions may reveal the maturity and source of the hydrocarbons as the trap has been seeing down into the generative basins during progressive burial. API and GORs of the petroleum in the inclusions are tentatively estimated from the fluorescence colour and the relative size of the gas bubble.

Traps in the western Haltenbanken region like Lavrans, Kristin, Trestakk and Smorbukk contain mainly petroleum of high maturity, yet in the compartmentalized Smorbukk field some medium maturity bitumen remains in particular reservoir sections. This is most likely due to strong compartmentahzation caused primarily by the lithological heterogeneities and secondarily by diagenetic poroperm modifications. 

In contrast, inclusions form more readily in carbonates and evaporites but these have an intrinsic tendency to experience recrystaUization or neomorphism. Leaking inclusions, precipitation of minerals inside the inclusion volume, and also necking-down phenomena may alter dramatically the internal pressure and volume ratios in the inclusions from the time of its initial entrapment, making microthermometric observations more difficult to translate directly into trapping temperature (cf. Roedder 1984 Ned-kvitne et al. 1993). This temperature is needed to decipher the time for entrapment by the use of the burial history for the trap (Karlsen et al. 1993). This type of application of inclusions as tape recorders for reservoir filling is in carbonates best done with the help of detailed cathodo-luminesence or backscatter techniques so that the various diagenetic events can be traced in detail. 

Still, it is difficult to foresee that inclusions can be used effectively to trace out much more detailed maturity traits in traps simply because the formation process in any given mineral is a continuous process and as oil dissolves oil, i.e. mixing results for biomarkers and other maturity and facies parameters. It is for these reasons that Karlsen et al. (1993) resorted to utilizing the quartz-plagioclase and the K-feldspar system, which are minerals with quite different diagenetic temperature responses, to get a more time resolved picture of the HC phases in the trap investigated. 

The trapping of pore fluids as sediments are buried may potentially preserve fluid from a time when ocean composition differed from the present, such as the last glacial period when salinity should have been greater than at present and the isotopic composition of water should have been heavier. However, pore water is an open system, and diffusion facilitates the re-equilibration between fossil pore water and bottom waters. Consequently, relict signals may be difficult to detect, even in the absence of any influence of diagenetic reactions. 

Increased erosion due to forest clear-cutting and widespread cultivation has increased riverine suspended matter concentrations, and thus increased the riverine particulate phosphorus flux. Dams, in contrast, decrease sediment loads in rivers and therefore diminish phosphorus-flux to the oceans. However, increased erosion below dams and diagenetic mobilization of phosphorus in sediments trapped behind dams moderates this effect. The overall effect has been a 50-300% increase in riverine phosphorus flux to the oceans above pre-agricultural levels. 

A large part of the liquid hydrocarbons and eventually also of the natural gas is formed in diagenetically mature zones above a peak in the formation of secondary porosity. Such a situation is very favourable as quartz sandstones with secondary porosity should be susceptible to trap these hydrocarbons. 

There are maity different types of petroleum traps. They are commonly classified into five main categories and associated subcategories as shown in Table II. These will now be defined and then described in turn Stractural traps are those caused by tectonic forces in the earth s crast. They thus include anticlines and fault traps. Diapiric traps are due to density contrasts in sedimentary rocks, normaUy evaporites and overpressured clays. Stratigraphic traps are due to depositional, erosional, or diagenetic processes. 

As previously defined, stratigraphic traps are due to de-positional, erosional, or diagenetic proeesses (Table II). Though the beds involved may be tilted from the horizontal, folding and faulting are absent in a pure stratigraphic... 

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