Relationship between stylolite morphology and the sealing potential of stylolite-bearing carbonate cap rocks

We analyzed the sealing effectiveness of cap rocks bearing different types of stylo-lites using a combination of petrographic, petrophysical, pore structure, and sealing capacity characterization techniques. This study was based on examples of carbonate cap rocks that seal ultradeep hydrocarbon reservoirs of the Tarim Basin (China). Sam-ples from both drill cores and their outcrop analogues were investigated to quantify how morphology influences the sealing capacity of different types of stylolite-bearing rocks. The study cap rocks consisted of mudstone, wackestone to packstone, grainstone, and dolomitic limestone. Four types of stylolites were identified: rectangular layer, seismo-gram pinning, suture and sharp peak, and simple wave-like types. The difference in the sealing capacity of carbonate cap rocks is attributed to their pore structure connec-tions and the types of stylolites they develop. Samples bearing simple wave-like stylolites showed the best sealing capacity, followed by those with rectangular layer and suture and sharp peak types, whereas carbonates host-ing seismogram pinning types had the lowest sealing capacity. The impact of stylolite seg-ments on the rock sealing properties, how-ever, differed from one segment to another. Rectangular layer-type stylolites could be di-vided into three distinct segments (with good, moderate, and poor sealing, respectively). Both the seismogram pinning and suture and sharp peak stylolite types were divided in two parts, with the former one having moderate and poor sealing and the latter exhibiting good and moderate sealing. The simple wave-like type had a good sealing capacity all along the pressure-solution seam. The most effec-tive sealing barriers for vertical fluid flow form when (1) calcite and siliceous cements are pervasively distributed in the vicinity of stylolites, forming highly cemented zones with lower porosity and permeability than their surrounding host rocks; (2) stylolites are enriched in insoluble residues; and (3) rare microfractures and dissolution vugs are found along the stylolites. This work provides useful examples for the prediction of the seal-ing potential of stylolite-bearing carbonate rocks according to stylolite morphology in other geologic settings.

Automated summary

We analyzed the sealing effectiveness of different types of stylolite-bearing cap rocks using a combination of characterization techniques. The study was based on carbonate cap rocks in the Tarim Basin. Samples from drill cores and outcrop analogues were used to quantify the influence of morphology on sealing capacity. The sealing capacity varied based on pore structure connections and stylolite types. Samples with simple wave-like stylolites showed the best sealing capacity, while those with seismogram pinning types had the lowest. The study provides examples for predicting sealing potential in other geological settings.