Petroleum emplacement inhibits quartz cementation and feldspar dissolution in a deeply buried sandstone

Abstract

Whether the emplacement of petroleum in sandstone reservoirs can preserve porosity during burial remains controversial. In the Kessog Field, UK Central North Sea, average porosities of the crestal sections of the fluvial-deltaic Pentland Formation reservoir can exceed 25 % despite burial to 4 km or more. The predicted porosity for the reservoir at this depth is only around 14 % based on regional data. Oil saturation data, thin-section point counts, grain-size and sorting measurements, reservoir pressure, and SEM images were combined to analyze the cause of the high reservoir porosity. Petroleum emplacement preventing cementation is the most likely mechanism for porosity preservation. Facies variation is not responsible, as the high-porosity sandstones from the crestal well are, in terms of average grain-size (fine-grained) and sorting coefficient (moderately well-sorted), nearly the same as the lower porosity sandstones from the flanks of the field (average porosity 13 -15%). Other potential porosity-preservation mechanisms, such as overpressure, grain-coats and feldspar dissolution can be discounted. The sandstones with high oil saturations are characterized by: 1) most porosity being primary as opposed to secondary; 2) there being 2 -5 % less quartz cement than in the water-saturated sandstones; 3) there being 2 -3 % more Kfeldspar and 2 -6 % less kaolin than the water-saturated counterparts. This study demonstrates that petroleum emplacement can effectively inhibit quartz cementation and K-feldspar transformation to kaolin in sandstone reservoirs.

Coupling relationship between reservoir densification and hydrocarbon emplacement in the Jurassic Badaowan Formation sandstone reservoirs of the southern Junggar Basin, China

“…The solid negative correlations between IGOI and the average capillary pressure and entry pressure also indicate growth of the lipophilicity of the pore channel, which decreases the capillary and displacement pressure. Similar phenomena have been reported by other scholars for tight sandstone in the Kuqa Basin and Ordos Basin(Lu et al, 2018;Xia et al, 2020).…”

“…Hydrocarbon emplacement, in most cases, suppressed the precipitation of carbonate cement (Xia, Wilkinson, & Haszeldine, 2020). Our above analysis showed that the first‐stage ferrocalcite cementation was associated with the first‐stage hydrocarbon emplacement (Figure 10; Table 5).…”

“…Hydrocarbon emplacement, in most cases, suppressed the precipitation of carbonate cement (Xia, Wilkinson, & Haszeldine, 2020).…”

Effect of Diagenetic Evolution and Hydrocarbon Charging on the Reservoir-Forming Process of the Jurassic Tight Sandstone in the Southern Junggar Basin, NW China

“…The high matrix content is an important reason for severe mechanical compaction. Under compaction, the matrix was squeezed into small pores and throats, which led to extensive plugging of primary pores and tremendous degradation of permeability [47]. Meanwhile, ductile particles form a large number of pseudo-matrices during compaction associated with meteoric water leaching, which further promoted the compaction-induced damage to the reservoir [47].…”

“…Under compaction, the matrix was squeezed into small pores and throats, which led to extensive plugging of primary pores and tremendous degradation of permeability [47]. Meanwhile, ductile particles form a large number of pseudo-matrices during compaction associated with meteoric water leaching, which further promoted the compaction-induced damage to the reservoir [47]. The primary porosity was computed empirically using the sorting coefficient that was obtained from the grain size analysis.…”