Porosity anomaly

Chapter 13 Organic Acids and Carbonate Stability, tbe Key to Predicting Positive Porosity Anomalies... 

If this sequence of carbonate reactions is coupled with parallel organic reactions, including generation and decarboxylation of organic acids and acid anions, a predictive, process-oriented model can be constructed for the carbonate reactions. The model consists of three operations (1) interpretation of reaction pathways (2) kinetic modeling of organic reactions and (3) simulation of rock/water interactions in either time or temperature space. Integrating these three operations allows us to predict zones of carbonate dissolution or optimum porosity enhancement (positive porosity anomalies) in source/reservoir rock systems. 

The most important links volumetrically in the reaction chain with respect to developing zones of enhanced porosity - positive porosity anomalies - are the carbonate cementation/decementation events (Fig. 1). The sequence of carbonate cementation/decementation events shown schematically in Fig. 1 has been noted by many investigators (see especially McBride 1977). Recent work includes Boles (1987), Taylor (1990), Moraes (1989), Yin and Surdam (1985), Jansa and Urrea (1990), and Surdam et al. (1989c,d). Although this list is by no means exhaustive, it does demonstrate that this series of progressive carbonate reactions has been observed in basins around the world by many different investigators. 

The formation of carbonate cements typically preserves the intergranular volume (IGV) present at the time of cement formation with subsequent progressive burial, the IGV will be anomalously high. If the carbonate cement is later destabilized and the carbonate is dissolved, the sandstone will be characterized by enhanced porosity, a positive porosity anomaly. Enhanced porosity, or a positive porosity anomaly, is defined herein as porosity at a specific depth that is greater than the porosity predicted at that depth by a compaction/depth curve for the corresponding sandstone lithology (see Sclater and Christie 1980 Pittman and Larese 1991). 

We believe that the probability of significantly enhancing effective porosity (generating a positive porosity anomaly) during burial diagenesis is largely a function of carbonate mineral stability. 

Log SI can be plotted versus time for each modeled layer, or it can be plotted against present-day depth. When log SI is positive, the mineral is oversaturated and will precipitate when log SI is negative, the mineral is undersaturated and will dissolve. Moreover, comparing log SI vs. present-day depth plots with porosity/depth plots allows the predicted mineral stabilities (potential porosity anomalies) to be tested directly. In this manner, a model of the evolution of porosity in a targeted clastic reservoir system can be accurately evaluated. 

Figure 16 is a diagram showing porosity/depth relations for sandstones of the Latrobe Group. The compaction/porosity loss curves are from Pittman and Larese (1991). The 50 and 75% quartz curves of porosity loss vs. depth (Fig. 16) establish a porosity baseline, based on the quartz content of each sandstone. These porosity loss vs depth curves are used to define porosity anomalies. 

Our interpretation of Fig. 24 is that there should be a significant dolomite dissolution event in the Gippsland Basin at a present-day depth from approximately 1200 to 2800 m. Over this depth interval, the log (AP/K) values are negative and dolomite typically is undersaturated. Points A, B, C, and D correspond to layers presently at 1500, 2800, 3600, and 4800 m depth. The next step in this discussion is to evaluate the quality of the porosity anomaly prediction for the 1200 to 2800 m present-day depth interval. 

A systematic and sequential set of carbonate reactions characterizes most clastic source/reservoir rock systems during progressive burial. Typically, this sequence of carbonate reactions with increasing thermal exposure (depth of burial) is as follows (1) formation of early carbonate cements that preserve IGV (2) dissolution of the early carbonate cements (3) formation of late carbonate cements (usually ferroan), again preserving IGV and (4) if temperatures are high enough, dissolution of the late carbonate cements. This carbonate reaction sequence is responsible for windows of opportunity for porosity enhancement (positive porosity anomalies). The zones of volumetrically important porosity enhancement are the result of carbonate dissolution. 

Fig. 3. Porosity anomalies shown by cross-bedded sands at unconformities in well A, from the Far East. Symbol type refer to wells...

The primary purpose of this section has been to show the possibilities for using density and area profile data to aid in the better understanding of gas-carbon reactions. In order to determine specific reaction rates and carbon dioxide concentrations at given penetrations, it has been necessary to make assumptions which can only be approximations to the truth. Several major anomalies in the results have been found, however. The calculated concentrations of carbon dioxide at the external surface of rods reacted at 1200 (Table VI) and 1305° are not in agreement with the known carbon dioxide concentrations. Clearly, more information is required on the variation of Deir with temperature and its variation with porosity produced at different reaction temperatures. It is feasible that at high temperatures, considerable porosity may be produced without increasing Deo to such a marked extent as found at 900°. Another anomaly is the non-uniformity of reaction found at 925°, when it would be expected that the reaction should be in Zone I. The preliminary assumption of a completely interconnecting pore system may not be valid. It should also be noted that neither the value of K in Equation (75) nor the low-temperature activa-... 

At all three sites, the six indirect indicators were found as listed in the Site 997 discussion. The similarity of the indicators in the three sites is exemplified by the chlorinity anomalies in the hydrate regions of Figure 7.22b. There is a minimum of approximately 1.4 vol%, 1.7% and 2.1% gas hydrate at Sites 994, 995, and 997, respectively assuming a low chlorinity baseline, and a sediment porosity of 50%. The amount of gas hydrate appears to increase from the ridge flank (Site 994) to the ridge crest (Site 997) with various indicators shown in Table 7.11. 

Fig. 14.13 Cartoon illustrating how gas hydrate formation increases the salinity of the adjacent interstitial pore fluid, and subsequent dissipation of the chloride anomaly via diffusion over time. A. Shows system before hydrate formation, sodium and chloride ions homogeneously distributed in the pore fluid. B. When gas hydrate forms, ions are excluded from the crystal lattice, and the pore fluids become saltier at the foci of hydrate formation. Right panel illustrates a 56 mM anomaly created by formation of gas hydrate that occupies 9% of the pore space. C. Over time the excess ions diffuse away, as illustrated by the diffusional decay model showing dissolved chloride profiles at 1,000 and 10,000 years. D. After 100,000 years, the chloride anomaly is smaller than that which can be detected with current analytical techniques. The 1-dimensional model assumes that the half width of the concentration spike to be 5 meters, a sediment porosity of 50% and the free solution diffusion coefficient for the chloride ion of 1.86 x 10 cm s" at 25 °C (modified from Ussier and Pauli 2001).

Common manufacturing defects include anomalies such as porosity, microcracking and delamination resulting from processing discrepancies as well as inadvertent edge... 

Fig. 16. Plot of porosity versus depth for the Latrobe Sandstone. + represents porosity values detected by the authors, while o represents porosity values obtained by Bodard et al. (1984), The area between the two curves, representing porosity loss due to compaction versus depth for a sandstone containing 50% quartz left-hand curve) and a sandstone containing 75% quartz right-hand curve), contains the base porosity values due to compaction predicted by Pittman and Larese (1991). The two horizontal lines outline positive anomalies, while the data points left of the base porosity area are negative anomalies...

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