The Impact of Diagenesis on the Reservoir Properties of the Carboniferous Sandstones of Western Pomerania (NW Poland)

Abstract

The aim of the study is to assess the effect of diagenesis on the reservoir properties of Carboniferous sandstones in Western Pomerania (NW Poland). The research focuses on Mississippian (Łobżonka Shale, Gozd Arkose, and Drzewiany Sandstone formations) and Pennsylvanian (Wolin, Rega, and Dziwna formations) rocks. A comparative analysis of the sandstones in the individual formations was carried out. The sandstone samples taken from 13 deep boreholes were studied petrographically (using a polarizing microscope, cathodoluminescence, and a scanning electron microscope), and petrophysical features were measured. The Carboniferous sandstones are represented mainly by quartz arenites ranging from very fine- to medium-grained and arkosic and lithic arenites from fine- to coarse-grained. The main diagenetic processes that affected the porosity and permeability of quartz arenites were compaction and cementation. Compaction reduced the primary porosity by an average of about 60% and cementation by about 40% in the Pennsylvanian sandstones. Primary porosity of arkosic and lithic arenites was affected mainly by compaction, cementation, and dissolution. Arkosic arenites have lost an average of 80% of their primary porosity as a result of mechanical compaction. The porosity of these sandstones increased due to the dissolution of mainly feldspar grains and the formation of secondary porosity. Among the Mississippian sandstones, quartz arenites of the Łobżonka Shale Formation exhibit unfavorable reservoir properties (porosity approx. 1%, impermeable). The volcaniclastic arkosic and lithic arenites of the Gozd Arkose Formation have poor reservoir qualities (porosity usually around 5%, mostly impermeable). The quartz arenites of the Drzewiany Sandstone Formation show the best reservoir properties (porosity of about 18%, permeability up to 1000 mD). The Pennsylvanian sandstones, quartz arenites of the Wolin and Rega formations, are characterized by good reservoir qualities (porosity approx. 10%, permeability up to 200 mD), while the Dziwna Formation sandstones show worse properties (porosity approx. 10%, often impermeable).

1. Introduction

The reservoir properties of rocks are influenced by diagenesis, a sum of the processes which affect a sediment after it has been laid down. Identifying these processes helps to assess their impact on rock porosity and permeability. Research on this topic has been conducted in various countries for many years (e.g., [1,2,3,4]).

Carboniferous deposits in Western Pomerania are found in the Kamień Pomorski–Koszalin zone (including an offshore area in the north, within the Polish part of the Baltic Sea) (Figure 1). The Carboniferous succession was either partly penetrated or drilled through in several tens of boreholes here, but only a few of the borehole sections are complete. The Carboniferous deposits represent the Mississippian (Tournaisian, Visean) and Pennsylvanian (Westphalian, Stefanian-?Autunian) (Figure 2). Within the Mississippian deposits, several informal lithostratigraphic units of formation rank have been distinguished, being of Tournaisian and Visean age, the oldest of which partially belonging to the Famennian [5,6,7]. In the north-eastern part of the Pomeranian area (Koszalin–Wierzchowo zone), six formations have been identified: the Sąpolno Calcareous Shale, Trzebiechowo Marl, Gozd Arkose, Kurowo Oolite, Grzybowo Calcareous Shale, and Drzewiany Sandstone (Figure 2). In the south-western part of the area (Laska–Czaplinek zone), four formations have been identified: the Sąpolno Calcareous Shale, Łobżonka Shale, Czaplinek Limestone, and Nadarzyce Shale (Figure 2). The Pennsylvanian succession, Westphalian and Stefanian-?Autunian in age, comprises three formations: the Wolin, Rega, and Dziwna [8] (Figure 2). The Carboniferous section is represented mainly by clastic rocks: sandstones, claystones, mudstones, and siltstones as well as by carbonates. The Carboniferous deposits rest unconformably on Devonian formations, and their top surface is overlain by Rotliegend or Zechstein deposits. Since the 1960s, work has been carried out in Western Pomerania to prospect for hydrocarbons in the Carboniferous succession. In 1965, natural gas accumulations were discovered in Mississippian sandstones near Wierzchowo [9]. In the following years, further accumulations were found: Daszewo-North and Białogard (Mississippian), and Wrzosowo and Gorzysław (Pennsylvanian) [9,10], among others. To this day, the Carboniferous deposits are of interest for many researchers (e.g., [11,12,13,14,15,16,17,18,19,20,21,22]). However, despite the research that has been conducted so far, the issue of the boundary between the Dziwna and Świniec formations—and therefore between the Pennsylvanian and Rotliegend—has not been definitely established. In the current lithostratigraphic division of the Carboniferous, the Dziwna Formation is included in the Pennsylvanian [22].

The aim of this study is to present the petrographic and mineralogical characteristics of the Carboniferous sandstones and the influence of diagenesis on the development of pore space within them. A comparative analysis of sandstones in the individual formations was carried out to assess their reservoir properties. The most prospective sandstones for hydrocarbon exploration were indicated. The data presented include rock material from selected, representative deep boreholes (Figure 1). The location and number of analyses conducted were taken into account when selecting the boreholes. Most of the results that were obtained from previous research of the Carboniferous deposits can be found in archived materials (e.g., [14,15,23]), but only few have been published (e.g., [11,12,13,16,17,18,19,24,25]). In earlier studies, the Dziwna Formation deposits were usually described separately [20]. This study focuses on sandstones belonging to the following formations: Łobżonka Shale, Gozd Arkose, and Drzewiany Sandstone (Mississippian), as well as Wolin, Rega, and Dziwna (Pennsylvanian) (Figure 2). In the other Carboniferous formations, sandstones are absent or represent a small percentage of the profile.

2. Geological Setting

The area of Western Pomerania extends westward from the Precambrian East European Craton (Figure 1). It is separated from the craton by the Koszalin fault zone, which is part of the transcontinental Teisseyre–Tornquist (TT) tectonic zone. According to [26], the TT zone forms the north-eastern boundary of an area of crystalline crust blocks with a Paleozoic sedimentary cover, approximately 100 km wide, referred to as the Trans-European Suture Zone (TESZ). The Koronowo–Margonin transverse deep fault zone separates the Pomeranian Block from the Kuyavian Block [27]. The eastern, erosional limit of Devonian and Carboniferous deposits is marked by the Koszalin–Chojnice–Toruń fault zone. The Variscan deformation front marks the southern boundary of the Devonian and Carboniferous epicontinental basins. The Carboniferous basin in the Pomeranian area is a continuation of the palaeogeographic setting of the pre-existing Devonian basin [6,7]. It is characterized by the continuous development of gradually decreasing subsidence from the Middle Devonian to the Late Visean (regressive stage). This was followed by the denudation of the area in the Namurian and Westphalian A and then by the final transgressive stage of the younger sedimentary cycle. The Carboniferous basin consists of several distinct areas with diachronous sedimentation and erosion. This is result of tectonic movements that occurred from the Erzgebirge phase to the Asturian phase (Namurian B to Westphalian D) [8]. At the turn of the Visean and Namurian, tectonic inversion associated with the Sudetic phase took place.

The Mississippian deposits are found in two areas: the Koszalin–Wierzchowo zone (north-east) and the Laska–Czaplinek zone (south-west) (Figure 1 and Figure 2). They occur at depths ranging from ca. 2.1 km to 6.0 km and reach a thickness of several hundred to approximately 1600 m (

Table 1. Boreholes and the number of samples studied.

Age	Formation	Borehole	Depth/Thickness [m]	Number of Samples
Pennsylvanian	Dziwna	Gorzysław 10	2859.0–2807.0/52.0	12
		Strzeżewo 1	3442.5–3199.0/243.5	16
		Wrzosowo 8	3180.0–3077.5/102.5	11
	Rega	Gorzysław 10	3010.0–2884.0/126.0	14
		Strzeżewo 1	3608.0–3442.5/165.5	4
		Wrzosowo 8	3310.0–3180.0/130.0	10
	Wolin	Gorzysław 10	3058.5–3010.0/48.5	1
		Sarbinowo 1	2494.0–2387.0/107.0	7
		Strzeżewo 1	3890.0–3608.0/282.0	13
Mississippian	Drzewiany Sandstone	Dygowo 1	3133.5–3218.5/85.0	19
		Koszalin IG 1	3012.0–2349.0/663.0	80
		Ustronie IG 1	3180.6–3105.5/75.0	6
	Gozd Arkose	Chmielno 1	3952.0–3578.0/374.0	15
		Daszewo 3P	3205.0–3260.0/45.0	20
		Dygowo 1	3831.0–3450.0/381.0	50
		Kłanino 3	3048.0–2456.0/592.0	80
	Łobżonka Shale	Czaplinek IG 1	6006.0–5703.5/302.5	12
		Moracz IG 1	4722.0–4581.0/140.0	17
		Zabartowo 2	4569.5–3900.0/669.5	8

). The area of occurrence of the Pennsylvanian deposits is smaller than that of the Mississippian deposits, being limited to the Sarbinowo area and the Kamień Pomorski–Trzebiatów zone (Figure 1 and Figure 2). They occur at depths ranging from ca. 2.0 km to 3.8 km, and their thickness varies from ca. 140 m to ca. 630 m (Table 1). The Mississippian deposits accumulated in a wide variety of sedimentary environments, ranging from open sea, clastic shelf, and shallow carbonate shelf to coastal and terrestrial environments [5,12,28]. The deepest-seated sandstones have been identified within the Łobżonka Shale and Gozd Arkose formations, while the Drzewiany Sandstone Formation sandstones occur at slightly shallower depths in the cores (Figure 2). The latter are partly contemporaneous with the Łobżonka Shale deposits, but younger than the Gozd Arkose deposits. The Pennsylvanian deposits mark a transition from marine conditions—tidal flats—to terrestrial conditions—fluvial and lacustrine environments [28]. All the way from the base to the top, sandstones occur in the Wolin, Rega, and Dziwna formations (Figure 2).

3. Materials and Methods

Sandstones representative for the Carboniferous of Western Pomerania were selected for the study from 13 deep boreholes (Figure 1; Table 1). Approximately 400 thin sections were analyzed under a Nikon polarizing microscope. Most of them were stained with Evamy’s solution for the preliminary identification of chemical composition of carbonates. The study was also aided by cathodoluminescence (CL) analysis performed on approximately 80 thin sections using a CCL 8200 mk 3 instrument from Cambridge Image Technology Ltd. (Great Britain) mounted on a Nikon Optiphot 2 (Japan) polarizing microscope tableResults of scanning electron microscope (SEM) studies of rocks, using JEOL JSM-35 (Japan) and LEO 1430 (Oberkochen, Germany) instruments incorporated with an EDS ISIS energy dispersive spectrometer were also used. Thirty chip samples and thirty-one thin sections were examined. The microlithofacies varieties of rocks were determined using the classification of sandstones by Pettijohn et al. [29] modified by Jaworowski [30] and Ryka and Maliszewska [31]. Porosimetric studies and permeability determinations on 98 selected sandstone samples were carried out at the Oil and Gas Institute in Kraków. An AutoPore9220 mercury porosimeter (USA) was used. The effective permeability coefficient was determined using nitrogen and calculated using the Darcy equation.

4. Results

4.1. Petrographic Characterization

Carboniferous sandstones of the studied formations differ in terms of the contents of detrital components and authigenic minerals. Classification of the sandstones is presented in a triangular diagram with end members of quartz (Q), feldspar (F), and rock fragments (L) (Figure 3).

4.1.1. Mississippian Deposits (Figure 2)

In the Laska–Czaplinek zone, sandstones occur in the Łobżonka Shale Formation within a series of black claystones and dark gray mudstones and siltstones. In the Koszalin–Wierzchowo zone, sandstones are found in the Gozd Arkose Formation and the Drzewiany Sandstone Formation and are accompanied by gray, red, and green siltstones, mudstones, and claystones.

The sandstones of the Łobżonka Shale Formation are red and gray in color and constitute a small proportion within a series of claystones, mudstones, and siltstones. They represent quartz arenites and quartz wackes, less frequently sublithic, ranging from very fine to fine-grained (Figure 3 and Figure 4A,B). They show an unoriented or directional fabric accentuated by the arrangement of flakes of clay minerals. The grains are mostly sub-rounded and well sorted. The main component of the grain framework is quartz, with monocrystalline quartz predominating over polycrystalline quartz, accompanied by small amounts of feldspar grains and mica flakes (muscovite, biotite) and lithoclasts (quartz schists, claystones, siliceous rocks, acidic volcanic rocks). Heavy minerals (zircon, tourmaline, amphibole) are rare. Locally, the rock contains bioclasts (foraminifera, crinoids, bivalves, brachiopods) and ooids. The sandstone is cemented with clay–ferrugineous matrix and cements, mainly quartz and carbonates (Fe-calcite, ankerite, siderite) (Figure 4A,B). There are also authigenic clay minerals (kaolinite, illite, chlorites), hematite, sulfates (anhydrite, gypsum), and pyrite (

Table 2. Authigenic minerals in the Carboniferous sandstones.

Authigenic Minerals	Formations
	Łobżonka Shale	Gozd Arkose	Drzewiany Sandstone	Wolin	Rega	Dziwna
kaolinite/dickite	+/?+	+/?+	+	+/+	+/+	+/+
illite	+	+	−	+	+	+
mixed-layer illite/smectite	−	+	−	+	+	−
chlorite	+ (Fe/Mg)	+	+ (Fe)	−	−	+ (Fe)
quartz/chalcedony	+	+/+	+	+	+	+
calcite	+ (Mn)	+ (Mn/Fe)	+ (Mn)	+ (Mn)	+ (Mn/Fe)	+ (Mn)
dolomite/ankerite	+/+	+/+	+/+	+/+	+/+	+/+
siderite	+	−	+	+	−	−
anhydrite/gypsum	+	+	+/+	+	+	+
barite	−	+	+	+	+	+
hematite	+	+	+	+	+	+
pyrite	+	+	+	+	+	+
feldspar	−	+	−	−	−	−

).

The sandstones of the Gozd Arkose Formation are pink, gray, red, and green in color. These are volcaniclastic sandstones characterized by the presence of volcanic material. The sandstones represent arkosic and subarkosic arenites and wackes which clearly predominate over lithic and sublithic arenites and wackes, ranging from fine- to coarse-grained (Figure 3 and Figure 4C–F). The sandstones are characterized by an unoriented or directional fabric, accentuated by the arrangement of flakes of clay minerals. The granular detrital material is mostly well sorted. The main components of the sandstone’s grain framework are feldspars, quartz, and lithoclasts. The feldspars belong to the potassium, potassium–sodium, and sodium varieties [15,24]. The quartz grains are monocrystalline and of volcanic origin (angular, embayed). The lithoclasts are dominated by fragments of volcanic rocks, mainly rhyolites and dacites. Numerous pseudomorphs after volcanic glass are found. There are also lithoclasts of plutonic rocks (granitoid type), sedimentary rocks (claystones, mudstones, siltstones), and metamorphic rocks (quartz schists). Minor amounts of micas, mainly biotite, are present. Heavy minerals are represented by zircon, tourmaline, apatite, and garnet. The cementing material is unevenly distributed in the sandstones. It consists of a matrix and pseudomatrix (a product of crushed and altered volcanic material) and cement. The cement is represented by carbonates (calcite, Fe/Mn-calcite, dolomite, Fe-dolomite, ankerite), sulfates (anhydrite, barite), authigenic clay minerals (illite, kaolinite, chlorites, illite/smectite mixed-layer minerals), authigenic quartz and authigenic feldspar, iron oxides and hydroxides, and pyrite (Table 2).

The sandstones of the Drzewiany Sandstone Formation are characterized by gray, light gray, or gray-pink colors. They represent quartz arenites and wackes, ranging from very fine- to fine-grained, less frequently medium-grained (Figure 3, Figure 4G,H and Figure 5A,B). The sandstones generally show an unoriented fabric. The grain material is angular or sub-rounded and well sorted. The basic component of the grain framework of the sandstones is quartz, with a predominance of monocrystalline over polycrystalline. In addition, small amounts of feldspar (mainly potassium feldspar), lithoclasts of metamorphic rocks (quartzite, quartz-mica schist) and sedimentary rocks, as well as mica flakes (muscovite) are present. Heavy minerals are represented by zircon, tourmaline, and rutile. The sandstones are cemented with a matrix and cements, which account for a small proportion of the rock. The cements include authigenic quartz and carbonates (Fe-dolomite, ankerite, Mn/Fe-calcite, siderite). In some places, greater quantities of authigenic clay minerals (kaolinite, chlorites), sulfates (anhydrite, gypsum, barite), hematite, and iron hydroxides were observed (Table 2).

4.1.2. Pennsylvanian Deposits (Figure 2)

Sandstones of the Wolin, Rega, and Dziwna formations are interbedded by claystones, mudstones, and siltstones. Locally in the Wolin Formation, coal intercalations and carbonate rocks occur. The sandstones of all these formations exhibit similar characteristics and have a similar diagenetic history.

The sandstones of the Wolin, Rega, and Dziwna formations are light gray, gray, pink, and cherry red in color. They represent very fine- to coarse-grained quartz arenites and wackes (Figure 3 and Figure 5C–H). Locally, there are sublithic arenites and wackes (Rega and Dziwna formations) and subarkosic arenites and wackes (Rega Formation). The fabric of the sandstones is unoriented or directional, accentuated by the arrangement of flakes of mica and clay minerals. The detrital grains are mostly sub-rounded and generally well sorted. Quartz is the main mineral component of the grain framework of the sandstones, with monocrystalline quartz grains predominating over polycrystalline ones. Feldspars occur in small quantities and are represented by potassium feldspars. Lithoclasts are a minor component of the rocks. These are dominated by clasts of igneous rocks (mainly acidic) and fragments of altered volcanic glass, with metamorphic rocks being less common and sedimentary rocks occurring only sporadically. Mica flakes (mainly muscovite, less frequently biotite altered into chlorite) are found in varying amounts. The accessory minerals are represented by zircon, less frequently rutile and titanite. Intergranular spaces in the sandstones are filled completely or partially with matrix and cement. The matrix is composed of detrital clay minerals, which in places form a mixture with quartz silt and ferruginous matter. The main components of the cement are authigenic quartz, authigenic clay minerals (kaolinite, illite), carbonates (Mn/Fe-calcite, dolomite, ankerite, siderite), hematite, and iron hydroxides. Sulfates (anhydrite and barite) are commonly found in small quantities (Table 2). Organic matter was also observed in places.

4.1.3. Authigenic Minerals

The occurrence of authigenic minerals in the sandstones of the individual formations is summarized in Table 2.

Clay minerals are represented by kaolinite/dickite, illite, mixed-layered illite/smectite minerals, and chlorites. Kaolinite occurs as pseudohexagonal crystals forming characteristic booklet structures (Figure 5D,E,G,H). It is typically dark blue under cathodoluminescence. In quartz sandstones, this mineral grows on authigenic quartz crystals. In arkosic sandstones, most kaolinite is a product of feldspar kaolinization. The effects of kaolinite illitization were observed in the Łobżonka Shale and Gozd Arkose formations and in the Pennsylvanian deposits. Vermiform kaolinite, blocky kaolinite, and blocky dickite have been identified. Dickite was found in the Pennsylvanian sandstones in deeper zones of the Carboniferous section, below 2600 m [19,20]. Dickite is also likely to occur in sandstones of the Łobżonka Shale and Gozd Arkose formations due to the deep burial of the rocks. In the Mississippian deposits, dickite was identified by Muszyński [11].

Illite fills the pore spaces in the rock, forming fibers or needles (Figure 5F,H). In addition, in arkosic–lithic sandstones of the Gozd Arkose Formation, it forms rims on grains, composed of plates adjoining each other (Figure 4C). Illite coexists with kaolinite, chlorites, and authigenic quartz. This mineral is one of the products of feldspar alteration.

Mixed-layered illite/smectite minerals were identified by X-ray studies in sandstones of the Gozd Arkose Formation [14,24] and the Wolin and Rega formations [32].

Chlorites represent ferruginous magnesium and ferruginous varieties. The Łobżonka Shale Formation sandstones contain Fe/Mg chlorites, whose plates are oriented randomly, directionally, or in rosettes. In the Gozd Arkose Formation sandstones, chlorites are the product of diagenetic alteration of feldspars, volcanic lithoclasts, and mica. Fe-chlorites have been identified in the Drzewiany Sandstone Formation. Chlorite plates either intergrow with authigenic quartz or they form small rosettes on the surface of quartz grains and crystals. In addition, chlorites form rims on quartz grains, limiting the development of quartz cement. Fe-chlorites in the Dziwna Formation are a product of kaolinite alteration, and they form thin rims on detrital grains.

Quartz occurs in the form of syntaxial overgrowths on quartz grains (Figure 4B,E and Figure 5A,C). It forms rhombohedral and prismatic individuals (Figure 5F). The boundary between detrital quartz and the overgrowth is often manifested by the presence of hematite and iron hydroxides, clayey and clayey–ferruginous rims, and fluid inclusions. Quartz cement is very clearly distinguished from detrital quartz in the CL image. Authigenic quartz is non-luminescent and dark brown or dark blue in color, in contrast to brown or blue quartz grains. In quartz arenites, quartz overgrowths develop on quartz grains, filling the pore space partially or completely. In the Mississippian sandstones, Biernacka [24] identified in places several generations of quartz overgrowths, while in the Pennsylvanian sandstones, two generations have been distinguished [19,20]. In the sandstones of the Drzewiany Sandstone Formation, the overgrowths are poorly developed and incomplete. Quartz cement developed most strongly in the deepest-buried sandstones, including those of the Łobżonka Shale Formation. The development of quartz overgrowths was inhibited by the formation of authigenic chlorite and kaolinite. In the arkosic–lithic arenites of the Gozd Arkose Formation, quartz cement is strongly developed at the contact points between quartz grains (Figure 4E). Quartz also fills secondary pores, including in feldspars. As a result of the dissolution of cement and quartz grains under pressure, microstylolite seams were formed. Homogenization temperature measurements in two-phase fluid inclusions in authigenic quartz indicate its precipitation in a temperature range of 77–183 °C [19]. Chalcedony was also found in the arkosic sandstones, in addition to quartz cement. It takes the form of microcrystalline aggregates within the clay cement.

Carbonates are represented mainly by dolomite and calcite and in some places by ankerite and siderite (

Table 3. Chemical compositions from microprobe analyses of the carbonates.

Age	Borehole	Depth (m)	Mg wt.%	Ca wt.%	Mn wt.%	Fe wt.%	MgCO3 mol%	CaCO3 mol%	MnCO3 mol%	FeCO3 mol%	Carbonate Type
Pennsylvania	Dziwna Formation
	Strzeżewo 1	3318.46	0.14	38.19	1.32	0.00	0.5	96.7	2.8	0.0	Mn-calcite
		3349.71	0.27	37.61	2.91	0.00	0.9	93.1	6.0	0.0	Mn-calcite
			0.33	37.90	1.92	0.00	1.2	94.8	4.0	0.0	Mn-calcite
		3402.06	0.25	37.41	2.11	0.00	0.9	94.6	4.5	0.0	Mn-calcite
			12.97	22.72	0.02	0.03	44.4	55.5	0.0	0.1	dolomite
	Rega Formation
	Wrzosowo 8	3187.6 *	0.05	39.48	0.81	0.03	0.2	98.0	1.7	0.1	Mn/Fe-calcite
		3192.4 *	0.45	37.18	1.95	0.77	1.6	92.8	4.1	1.6	Mn/Fe-calcite
			0.28	37.85	1.56	0.20	1.0	95.3	3.3	0.4	Mn/Fe-calcite
		3193.6 *	0.00	38.06	2.40	0.00	0.0	95.0	0.5	0.0	Mn-calcite
			0.00	39.83	2.51	0.00	0.0	97.8	3.4	0.0	Mn-calcite
	Wolin Formation
	Gorzysław 10	3043.3 *	12.92	19.79	2.72	0.00	44.9	49.4	5.7	0.0	dolomite
	Sarbinowo 1	2455.3 *	10.56	22.84	0.89	0.00	38.3	59.7	2.0	0.0	dolomite
	Strzeżewo 1	3463.1 *	0.00	36.74	1.51	0.00	0.0	96.6	3.4	0.0	Mn-calcite
		3888.5 *	7.42	21.08	1.52	9.28	25.5	52.2	3.2	19.1	ankerite
Mississippian	Drzewiany Sandstone Formation
	Dygowo 1	3157.4	0.34	39.61	0.91	0.26	1.2	96.5	1.8	0.5	Mn-calcite
	Koszalin IG 1	2577.7	11.79	22.78	0.56	1.42	39.4	54.3	1.1	5.2	Fe-dolomite
		2675.8	0.46	44.49	0.99	0.59	1.4	95.8	1.8	1.0	Mn/Fe-calcite
			7.0	25.50	1.05	9.99	22.0	57.4	2.0	18.6	ankerite
			7.81	23.58	1.98	4.41	27.4	59.3	4.1	9.2	Fe-dolomite
			6.70	21.78	0.64	9.08	23.9	55.6	1.3	19.2	ankerite
			12.03	0.83	1.72	22.19	44.9	2.2	3.9	49.0	pistomesite
		2687.3	7.29	23.94	0.92	8.75	24.2	56.8	1.8	17.2	ankerite
			7.32	0.34	1.93	35.41	24.7	0.8	3.9	70.7	sideroplesite
		2803.6	12.35	22.44	0.43	1.14	42.1	54.7	0.9	2.3	Fe-dolomite
	Gozd Arkose Formation
	Daszewo 3P	3207.7	11.49	22.81	0.44	0.41	40.4	57.8	0.9	0.9	dolomite
		3219.0	12.87	24.38	0.43	0.08	42.0	57.0	0.8	0.2	dolimite
		3228.2	0.32	40.78	0.36	0.60	1.1	97.0	0.7	1.2	Fe/Mn-calcite
			11.05	24.40	1.02	3.79	35.0	55.8	2.0	7.2	Fe-dolomite
	Dygowo 1	3459.6	0.00	42.05	0.52	0.29	0.0	98.4	1.0	0.6	Mn/Fe-calcite
			7.71	28.49	1.25	5.61	24.0	63.4	2.3	10.3	ankerite
		3513.8	11.03	26.74	1.95	3.60	33.6	56.6	3.5	6.3	Fe-dolomite
			13.87	25.33	0.59	0.34	40.5	52.8	6.1	0.6	dolomite
		3257.5	13.79	26.64	0.77	0.48	41.1	56.7	1.4	0.8	dolomite
		3634.8	0.31	42.16	0.00	0.58	1.0	98.9	0.0	1.1	Fe-calcite
			8.46	24.84	0.00	3.91	29.7	62.2	0	8.1	Fe-dolomite
		3651.0	0.41	44.04	0.11	0.59	1.2	97.5	0.2	1.1	Fe-calcite
			9.97	24.68	0.15	4.45	32.8	58.2	0.3	8.7	Fe-dolomite
		3792.0	0.35	43.16	0.30	0.27	1.1	97.9	0.5	0.5	Mn/Fe-calcite
			13.59	25.36	0.32	1.36	41.6	55.4	0.6	2.4	Fe-dolomite
			9.98	24.36	0.46	7.22	31.2	54.6	0.8	13.4	ankerite
	Kłanino 3	2506.6	12.39	22.19	0	0.57	43.4	55.5	0.0	1.1	dolomite
			12.55	22.80	0.15	0.69	42.8	55.7	0.3	1.4	dolomite
		2512.8	0.34	39.58	0.00	0.00	1.2	98.8	0.0	0.0	calcite
			9.89	21.97	0.26	1.31	37.5	59.1	0.5	2.9	Fe-dolomite
	Kłanino 3		0.34	40.06	0.12	0.00	1.2	98.6	0.2	0.0	calcite
		2525.5	0.44	46.25	0.00	0.00	1.4	98.6	0.0	0.0	calcite
			9.21	22.65	0.27	1.74	34.6	60.9	0.6	3.9	Fe-dolomite
			8.07	22.25	0.72	5.12	29.4	58.3	1.5	10.8	ankerite
		2563.9	0.39	40.58	0.38	0.36	1.3	97.2	0.8	0.7	Mn/Fe-calcite
			10.74	22.96	0.42	2.57	37.2	56.7	0.9	5.2	Fe-dolomite
			9.57	23.53	0.34	1.03	35.2	61.9	0.7	2.2	Fe-dolomite
			7.99	21.87	0.41	3.10	31.1	60.8	1.0	7.1	Fe-dolomite
			7.71	22.34	0.32	3.78	29.5	61.2	0.8	8.5	Fe-dolomite
		2672.4	10.10	22.18	0.26	0.65	38.2	59.8	0.6	1.4	dolomite
			7.52	21.62	0.86	6.45	27.5	56.5	1.9	14.1	ankerite
		2694.2	12.04	23.04	0.50	1.66	41.2	54.7	0.9	3.2	Fe-dolomite
		2755.0	14.34	28.10	0.00	0.00	41.7	58.3	0.0	0.0	dolomite
			13.94	28.07	0.00	2.69	39.2	56.3	0.0	4.5	Fe-dolomite
		2778.0	11.52	22.93	0.26	2.65	40.3	57.3	0.5	5.5	Fe-dolomite
		2996.2	6.51	22.25	0.93	7.69	25.1	61.3	2.1	11.5	ankerite
			10.65	23.45	0.16	0.00	38.8	60.9	0.3	0.0	dolomite
			10.35	23.30	0.40	1.14	37.1	59.6	0.8	2.5	Fe-dolomite
		3038.9	9.52	22.56	0.00	1.72	35.7	60.4	0.0	3.9	Fe-dolomite
	Łobżonka Shale Formation
	Moracz IG 1	4649.3	0.25	41.11	0.24	0.26	0.9	98.2	0.5	0.5	Mn/Fe-calcite
			0.61	41.61	0.00	0.44	2.0	97.2	0.0	0.8	Fe-calcite
		4714.2	0.00	39.01	0.00	0.39	0.0	99.2	0.0	0.8	Fe-calcite
			0.15	40.34	0.00	0.98	0.5	97.6	0.0	1.9	Fe-calcite
			7.28	22.39	0.33	8.50	25.6	56.1	0.7	17.6	ankerite

* data from Reference [19].

; Figure 6). Calcite forms spar and microspar crystals filling the pore space in the sandstones (Figure 4A,B,H). When reacted with Evamy’s solution, calcite turns either pink-purple, indicating iron admixtures, or less frequently red. Under cathodoluminescence (CL) microscopy, calcite ranges from yellow to orange in color. In some places, areas of darker and lighter hues are visible under CL, indicating fluctuations in the manganese to iron ratio. The chemical composition of calcite is 92.8–98.2 mol% CaCO₃, 0–1.3 mol% MgCO₃, 0–1.6 mol% FeCO₃, and 0.2–4.1 mol% MnCO₃ (Table 3; Figure 6). It mostly represents Mn-calcite and Mn/Fe-calcite. Traces of dissolution were observed in the calcite crystals. Measurements of the homogenization temperature of two-phase inclusions in the calcite cement indicate its crystallization within a temperature range from 95 to 165 °C [19].

Dolomite occurs in the form of automorphic crystals (rhombohedrons) filling the intergranular spaces (Figure 4D,E,H). Dolomite crystals are often impregnated with hematite. In thin sections treated with Evamy’s solution, dolomite does not stain, which indicates the absence or low admixture of iron. The chemical composition of dolomite is as follows: 49.4–60.5 mol% CaCO₃, 34.0–40.2 mol% MgCO₃, 0–7.9 mol% FeCO₃, and 0.8–5.7 mol% MnCO₃ (Table 3; Figure 6). It represents dolomite and Fe-dolomite. In the CL image, dolomite exhibits colors ranging from orange-red to brownish, often displaying a distinct zonal crystal structure. The luminescence color in individual zones depends on the ratio of iron to manganese. Dolomite replaces grains of feldspars, lithoclasts and micas, as well as quartz and feldspar cements. Additionally, in arkosic–lithic arenites, this mineral is a product of replacement of calcite ooids. Traces of dolomite dissolution are visible under the SEM. Homogenization temperature measurements in two-phase fluid inclusions in dolomite crystals indicate their crystallization in a temperature range of 55–150 °C [16,19].

Ankerite occurs as automorphic crystals, forming porous cement. It often forms the outer zones of dolomite crystals, constituting their youngest phase. When viewed in thin sections stained with Evans’ fluid, its dark blue color stands out due to its significant iron content. The ankerite is composed of up to 19.1 mol% FeCO₃ and up to 3.2 mol% MnCO₃ (Table 3; Figure 6). It replaces grains of feldspars and lithoclasts, as well as dolomite and quartz cements. Traces of its dissolution are visible under the SEM.

Siderite occurs sporadically in the sandstones of the Łobżonka Shale, Drzewiany Sandstones, and Wolin formations. It forms as either very fine crystals (micrite, microspar) in the pore space or spherulites 0.08–0.15 mm in size. The chemical composition of the mineral corresponds to sideroplesite and pistomesite (Table 3; Figure 6).

Sulfates are represented mainly by anhydrite, gypsum, and barite. They have been identified at various depths in the Carboniferous section. Anhydrite occurs in the form of polycrystalline plates, locally forming nest-like aggregates (Figure 4D,H). The mineral often fills the pore spaces within feldspar and carbonate grains (bioclasts, ooids), which formed as a result of the dissolution of these grains. In addition, it replaces feldspar and lithoclast grains, as well as quartz and carbonate cements. Anhydrite is also a component that fills fractures. Chemical composition analyses did not reveal any admixtures in anhydrite.

Gypsum occurs in association with anhydrite, forming elongated plates. In quartz arenites of the Drzewiany Sandstone Formation, fibrous gypsum has been identified. It probably formed as a result of late diagenetic hydration of anhydrite [20].

Barite most often forms automorphic bars that crystallize in intergranular spaces (Figure 5E). In places, it coexists with anhydrite and carbonates. Chemical analyses show admixtures of strontium in the barite, up to a maximum of 4.6 wt.%.

Hematite and iron hydroxides are in places the main components of the cementing material in the Pennsylvanian sandstones. Locally, larger amounts of hematite have been identified in sandstones from the Łobżonka Shale and Gozd Arkose formations. Hematite and iron hydroxides not only fill the intergranular spaces of the rock and impregnate clay and dolomite cements but also accumulate in feldspar grains and volcanic lithoclasts. They form rims on detrital grains (Figure 4G and Figure 5G) as well as fill microstylolite seams. In places, hematite forms nest-like aggregates in which quartz grains are embedded. Another form of crystalline hematite is very fine balls, 0.002–0.004 mm in diameter, clearly visible under the SEM (Figure 5D).

Pyrite occurs in small quantities in the form of octahedrons and framboidal aggregates. It is visible in carbonate and sulfate cements and among clay minerals. Pyrite also encrusts carbonaceous plant remains and bioclasts.

Feldspar forms overgrowths on feldspar grains in arkosic sandstones of the Gozd Shale Formation (Figure 4E). Authigenic feldspar can be clearly identified because it has a different optical orientation to feldspar grains. Chemical analyses of authigenic feldspar reveal a potassium feldspar composition.

4.2. Pore Space

The Carboniferous sandstones show primary and secondary porosity. Primary porosity in the rock has been reduced as a result of mechanical compaction and cementation, among other factors. The partial preservation of primary porosity in the sandstones was favored by the presence of early quartz cement, which forms overgrowths around quartz grains (Figure 4G and Figure 5A–C,E,G). The quartz overgrowths have strengthened the rock, limiting the effects of mechanical compaction. Secondary porosity developed as a result of the dissolution of feldspar grains and volcanic rock clasts (Figure 4F and Figure 5B,C,G). In addition, microporosity between crystals of authigenic clay minerals, such as kaolinite (Figure 5E,G) or illite, is common. Due to the microscopic size of these pores, they do not significantly increase the total porosity and permeability of the rock. Primary porosity dominates in sandstones of the Drzewiany Sandstone, Wolin, Rega, and Dziwna formations [19]. Secondary porosity accounts for a small percentage in these formations but is very well developed in the Gozd Arkose Formation sandstones. Porosity, permeability, and porosimetry data (e.g., the content of pores >1 µm, threshold diameter, and hysteresis) are the parameters of sandstone’s pore space, which determine its reservoir qualities. The threshold diameter value determines the pore size for which continuous fluid flow through the sample is observed. Hysteresis is an indicator of filtration properties of the rock; the higher the value, the better the properties.

The Łobżonka Shale Formation sandstones are characterized by a porosity ranging from 0.4 to 3.8% (average 1.3%), and they are impermeable (

Table 4. Petrophysical features of selected sandstone samples.

Age	Formation	Borehole	Depth	Grain	Effective	Bulk	Porosimeter	Porosimeter	Average	Specific	Pores	Threshold	Hysteresis	Permeability
				Density	Density	Porosity	Density	Porosity	Capillary	Surface	>1 µm	Diameter
			[m]	[g/cm3]	[%]	[g/cm3]	[g/cm3]	[%]	[µm]	[m2/g]	[%]	[µm]	[%]	[mD]
Pennsylvanian	Dziwna	Strzeżewo 1	3317.50	2.62	11.20	2.71	2.35	10.24	0.11	1.56	53.00	2.50	68	n.p.
			3321.50	2.63	8.11	2.69	2.43	7.63	0.54	0.23	57.00	3.00	70	n.p.
			3395.10	2.60	10.45	2.70	2.36	9.50	0.22	0.74	62.00	5.00	72	2.33
			3402.20	2.67	13.50	2.75	2.34	12.48	0.13	1.62	62.00	4.00	67	7.44
			3412.20	2.63	3.52	2.70	2.55	3.28	0.06	0.87	21.00	0.20	32	n.p.
			3413.20	2.55	5.70	2.68	2.43	5.05	0.45	0.19	47.00	1.50	74	n.p.
			3431.10	2.46	0.41	2.75	2.45	0.32	0.00	0.00	n.d.	n.d.	n.d.	n.p.
	Rega	Gorzysław 10	2896.20	2.60	12.95	2.67	2.29	12.12	0.88	0.24	82.00	35.00	26	40.7107
			2973.60	2.59	11.48	2.67	2.32	10.69	0.51	0.37	70.00	15.00	53	<0.1
		Wrzosowo 8	3192.40	2.64	9.57	2.69	2.40	9.17	0.20	0.76	56.00	3.00	54	1.2975
			3202.80	2.57	12.67	2.70	2.28	11.21	1.17	0.17	81.00	30.00	33	n.d.
	Wolin	Sarbinowo 1	2390.10	2.61	20.25	2.68	2.12	18.90	0.52	0.69	85.00	30.00	35	230.9008
			2424.50	2.70	18.38	2.67	2.19	18.91	0.34	1.00	74.00	25.00	32	28.6291
		Strzeżewo 1	3648.50	2.64	6.86	2.68	2.47	6.61	0.23	0.47	32.00	3.00	50	n.d.
			3689.70	2.67	5.59	2.44	2.33	4.55	0.83	0.09	71.00	6.00	65	7.52
			3724.20	2.62	10.10	2.66	2.36	9.67	0.58	0.28	70.00	9.00	57	4.2415
Mississippian	Drzewiany Sandstone	Dygowo 1	3149.90	2.68	12.29	2.67	2.34	12.17	1.19	0.17	90.00	25.00	10	77.40
			3150.50	2.66	13.03	2.61	2.29	12.38	2.56	0.08	91.00	25.00	9	92.12
			3164.40	2.68	10.64	2.59	2.33	9.76	0.97	0.17	85.00	9.00	34	39.60
			3180.20	2.67	13.89	2.55	2.24	12.43	0.72	0.31	80.00	15.00	50	66.56
			3202.70	2.69	7.95	2.56	2.40	7.03	0.81	0.14	76.00	10.00	30	24.10
		Koszalin IG 1	2340.70	2.73	5.91	2.14	2.06	3.73	0.24	0.30	50.00	n.d.	25	335.30
			2354.00	2.72	9.44	2.66	2.42	8.91	0.46	0.32	84.00	25.00	10	57.50
			2360.90	2.74	25.56	2.63	2.03	23.05	3.67	0.12	98.00	30.00	2	1084.25
			2378.20	2.77	7.61	2.15	2.05	4.72	0.17	0.54	54.00	n.d.	44	476.48
			2414.70	2.71	28.98	2.58	1.92	25.57	1.06	0.50	92.00	25.00	7	n.d.
			2449.50	2.69	26.77	2.66	1,96	25.99	4.61	0.11	96.00	30.00	4	1091.90
			2463.25	2.69	12.36	2.67	2.34	12.12	0.37	0.56	89.00	12.00	20	56.64
			2515.00	2.71	3.83	2.70	2.60	3.80	0.10	0.58	24.00	2.50	55	n.p.
			2538.50	2.72	27.25	2.62	1.97	24.82	0.86	0.59	97.00	35.00	3	971.18
			2568.85	2.72	26.28	2.63	2.00	24.11	0.72	0.67	94.00	27.00	8	457.42
			2597.80	2.74	20.45	2.62	2.14	18.19	3.29	0.10	94.00	30.00	3	187.19
			2632.40	2.68	19.67	2.64	2.14	18.92	1.45	0.24	93.00	30.00	7	154.30
			2657.00	2.70	17.69	2.59	2.18	15.94	0.61	0.48	78.00	15.00	36	75.80
			2665.90	2.71	19.61	2.69	2.18	19.23	1.21	0.29	91.00	20.00	9	156.70
		Ustronie IG 1	3108.10	2.60	22.45	2.72	2.08	20.03	1.76	0.22	85.00	35.00	5	199.80
			3112.50	2.70	22.29	2.60	2.07	20.25	2.37	0.17	82.00	30.00	6	154.08
			3126.90	2.69	15.61	2.58	2.22	14.06	0.49	0.52	88.00	15.00	24	18.39
			3152.70	2.74	12.47	2.58	2.30	10.83	0.39	0.49	82.00	15.00	27	75.23
			3159.40	2.69	23.09	2.79	2.08	25.65	0.52	0.95	95.00	20.00	25	290.47
			3171.20	2.71	14.41	2.49	2.19	11.85	0.42	0.52	85.00	8.00	55	54.77
			3171.40	2.70	10.55	2.49	2.27	8.72	0.76	0.20	88.00	10.00	48	23.01
	Gozd Arkose	Chmielno 1	3606.60	2.67	13.02	2.55	2.25	11.55	0.10	1.99	19.00	2.00	78	n.p.
			3614.30	2.67	10.66	2.60	2.34	9.00	0.09	1.93	15.00	1.50	72	n.p.
			3662.50	2.69	7.44	2.61	2.43	6.88	0.07	1.72	5.00	0.70	74	n.p.
			3899.70	2.27	8.92	2.57	2.37	7.79	0.07	1.77	65.00	6.00	51	15.09
			3910.40	2.67	7.14	2.59	2.42	6.60	0.07	1.46	26.00	2.00	69	n.p.
			3990.50	2.72	0.62	2.57	2.56	0.54	4.09	0.00	85.00	85.00	n.d.	n.p.
		Daszewo 3P	3214.20	2.65	5.42	2.60	2.46	5.15	0.06	1.50	50.00	2.00	36	n.p.
			3220.10	2.80	3.07	2.62	2.59	2.59	4.75	0.00	94.00	n.d.	52	n.p.
		Dygowo 1	3505.20	2.65	5.55	2.60	2.46	5.27	0.16	0.55	76.00	6.00	30	n.p.
			3513.80	2.67	4.61	2,59	2.48	4.27	0.05	1.32	34.00	2.00	43	n.d.
			3527.50	2.66	3.98	2.62	2.52	3.83	0.05	1.11	33.00	2.00	55	n.d.
			3641.10	2.67	2.05	2.62	2.57	1.95	0.14	0.21	76.00	n.d.	35	n.d.
			3657.30	2.71	1.19	2.73	2.70	1.21	0.35	0.05	71.00	n.d.	21	n.p.
			3664.50	2.68	2.93	2.65	2.57	2.86	1.23	0.04	74.00	4.00	35	n.p.
			3665.90	2.67	3.04	2.62	2.54	2.90	0.10	0.45	55.00	2.50	41	n.d.
			3677.90	2.68	4.16	2.65	2.54	4.04	0.25	0.25	80.00	n.d.	20	n.d.
			3717.10	2.68	3.57	2.64	2.55	3.42	0.73	0.07	69.00	5.00	26	n.p.
			3722.40	2.74	4.48	2.74	2.62	4.48	0.05	1.34	22.00	1.00	63	n.d.
			3755.10	2.64	5.96	2.56	2.42	5.51	0.13	0.72	57.00	3.00	55	0.01
			3787.50	2.64	1.45	2.58	2.53	1.97	0.30	0.10	64.00	1.50	30	n.p.
			3792.00	2.63	2.07	2.62	2.56	2.05	0.13	0.25	56.00	2.00	45	n.p.
			3795.30	2.76	1.72	2.68	2.63	1.58	0.13	0.18	26.00	0.60	61	n.p.
			3825.70	2.77	3.20	2.68	2.60	2.94	0.04	1.07	25.00	0.60	49	n.d.
		Kłanino 3	2465.30	2.68	0.42	2.58	2.57	0.39	0.02	0.29	7.00	0.05	95	n.p.
			2473.70	2.65	7.33	2.62	2.43	7.12	0.06	1.91	19.00	1.00	65	n.p.
			2477.60	2.64	8.74	2.61	2.39	8.47	0.07	2.13	18.00	0.40	61	n.p.
			2501.10	2.73	8.33	2.72	2.50	8.25	0./11	1.16	43.00	2.00	75	n.d.
			2515.50	2.70	6.64	2.60	2.44	6.00	0.12	0.82	42.00	4.00	67	n.p.
			2523.50	2.67	6.29	2.59	2.44	5.80	0.24	0.39	56.00	3	78	n.p.
			2523.70	2.65	8.94	2.59	2.37	8.44	0.16	0.91	60.00	2	73	0.1
			2568.70	2.71	4.42	2.65	2.54	4.17	0.03	1.98	11.00	0	62	n.p.
			2585.40	2.67	3.76	2.60	2.51	3.51	0.02	2.40	13.00	0	60	n.d.
			2634.80	2.71	4.11	2.61	2.51	3.74	0.06	0.97	23.00	1	53	n.d.
			2644.50	2.70	3.53	2.66	2.57	3.39	0.04	1.51	33.00	2	55	0.1
			2681.20	2.72	11.28	2.61	2.34	10.09	0.07	2.42	10.00	0.60	72	n.p.
			2684.80	2.65	6.41	2./62	2.45	6.22	0.06	1.84	49.00	n.d.	57	n.p.
			2694.20	2.72	5.87	2.67	2.52	5.59	0.04	2.21	14.00	0.20	66	n.p.
			2726.60	2.62	14.35	2.55	2.21	13.41	0.16	1.48	57.00	8.00	65	3.4
2764.60			2.71	11.70	2.66	2.37	11.14	0.09	2.11	14.00	1	69	n.p.
2996.20			2.65	10.77	2.59	2.33	10.16	0.14	1.26	33.00	4.00	77	n.p.
Łobżonka Shale	Czaplinek IG 1	5746.00	2.74	0.87	2.73	2.71	0.86	0.11	0.12	40.00	0.20	44	n.d.
		5900.00	2.70	0.69	2.66	2.64	0.66	0.00	0.00	n.d.	n.d.	n.d.	n.p.
	Moracz IG 1	4623.40	2.70	1.29	2.66	2.63	1.24	0.04	0.43	35.00	0.20	57	n.d.
		4703.00	2.69	1.24	2.68	2.65	1.23	3.69	0.01	83.00	8.00	10	n.p.
	Zabartowo 2	3971.70	2.64	3.80	2.66	2.54	0.25	0.63	0.23	17.00	0.90	62	n.p.
		4400.70	2.71	0.45	2.70	2.69	0.45	0.10	0.07	70.00	0.15	25	n.p.
		4504.20	2.69	1.01	2.64	2.61	0.96	0.13	0.11	90.00	n.d.	35	n.p.
		4549.40	2.68	0.74	2.66	2.64	0.73	0.05	0.24	46.00	0.10	48	n.p.

n.d.—no data; n.p.—non-permeable.

). The percentage of >1 μm pores varies from 17 to 90% (average 54%). The threshold diameter is in the range of 0.1–8.0 μm, most frequently <1.0 μm. The hysteresis value ranges from 25 to 62%, usually ~40%.

The Gozd Arkose Formation sandstones have a porosity ranging from 0.4 to 14.3% (average 5.9%) and they are mostly impermeable (Table 4). A permeability of 15.09 mD was measured in a sample from the Chmielno 1 borehole. The percentage of >1 μm pores varies from 5 to 94% (average 47%). The threshold diameter is in the range of 0–8.0 μm (average 1.5 μm). The hysteresis value ranges from 21 to 95% and is usually ~56%.

The Drzewiany Sandstone Formation sandstones are characterized by a porosity ranging from 3.8 to 29.0% (average 15.8%), and their permeability is from 18.39 to 1084.25 mD (average 200.00 mD) (Table 4). The percentage of >1 μm pores varies from 24 to 100% (average 84%). The threshold diameter is between 2.5 and 35.0 μm (average 20.0 μm). The hysteresis value ranges from 2 to 55%, usually ~23%.

Sandstones of the Wolin and Rega formations have a porosity ranging typically from 5.6 to 20.2% (average 11.0%) and their permeability is between <0.1 and >230 mD (Table 4). The percentage of pores larger than 1 µm varies from 32 to 85% (average 70%). The threshold diameter is in the range of 3.0–35.0 μm (average 18.0 μm).The hysteresis value varies from 26 to 65% (average 43%).

The Dziwna Formation sandstones have a porosity ranging from 0.4 to 13.5% (average 7.5%). Their permeability is a few mD, or they are impermeable (Table 4). The percentage of >1 μm pores varies from 21 to 62% (average 50%). The threshold diameter is in the range of 0.2–5.0 μm (average 2.7 μm). The hysteresis value ranges from 26 to 72%, usually ~64%.

5. Discussion

5.1. Formation of Authigenic Minerals Against the Background of Diagenesis History

Figure 7 summarizes the paragenetic sequence of the Mississippian and Pennsylvanian sandstones in Western Pomerania. Eo- and mezodiagenesis have been distinguished, and a temperature of 50 °C has been assumed as the boundary between them [33,34]. In the diagenetic history of the deposits, the following processes took place: compaction, cementation, replacement, dissolution and alteration. Shortly after the deposition of granular material, mechanical compaction began to take effect. One of the earliest authigenic minerals to crystallize as a result of the dissolution and alteration of volcanic glass was smectite [24]. Next, iron hydroxides and hematite formed as a result of either crystallization from solutions or the decomposition of unstable, iron-containing detrital minerals. Some of the hematite formed at a later stage of diagenesis, following the formation of quartz overgrowths. Locally, under reducing conditions, siderite crystallized, presumably in a temperature range of 15 to 40 °C [35]. Vermiform kaolinite also had an early origin, forming in an acidic environment [36] at a temperature of 25–50 °C [37]. The aluminum and silicon ions required for the formation of kaolinite were released as a result of the alteration of mica and feldspar grains due to the action of meteoric water [38]. During early diagenesis, the alteration of smectite into mixed-layered illite/smectite minerals and the formation of quartz and potassium feldspar overgrowths were initiated. The development of overgrowths strengthened the grain framework of the sandstone, which limited the effect of mechanical compaction and locally contributed to the preservation of part of their original porosity. During this period, barite crystallized locally. Fe/Mg chlorites were formed, with the Fe²⁺ and Mg²⁺ ions originating from the illitization of smectite. At a later stage of diagenesis, vermiform kaolinite was replaced by blocky kaolinite, which forms at temperatures ranging from 50 to 80 °C [37]. It developed either as a result of the alteration of vermicular kaolinite due to an increase in burial depth (e.g., [39,40]) or precipitated directly from pore solutions circulating in the rock [41]. The development of authigenic quartz continued, which in places almost completely filled voids in the pore spaces of the sandstones. The increase in quartz cementation was noticeable particularly at greater depths. The crystallization temperature of quartz was estimated at approximately 70–180 °C [19]. In the early stages of authigenic quartz formation, silica was probably derived from silica-rich meteoric waters and from the dissolution and alteration of feldspars and volcanic lithoclasts. At greater depths, pressure dissolution at quartz grain contacts, replacement of quartz by carbonates, and illitization of kaolinite may have been significant. Locally, authigenic albite formed, probably due to both the dissolution of plagioclase grains releasing sodium [42,43] and the alteration of mica [43] or as a result of the alteration of smectite into illite [44]. Albitization is the result of a dissolution–replacement process that occurs at temperatures between 75 and 100 °C [43,45,46]. Subsequently, carbonate cements precipitated: calcite and Fe-calcite, dolomite, Fe-dolomite, Mn/Fe-calcite, and ankerite. The sources of calcium for calcite and iron and magnesium for dolomite could have been marine waters, calcite grains, and smectites altered into illite [47,48]. The crystallization temperature of calcite and Mn/Fe-calcite was determined to be approximately 47–70 °C [14] and 95–165 °C [19], respectively, and that of dolomite was in the range of 55–150 °C [16,19]. Of the carbonate cements of the Carboniferous sandstones, ankerite formed last. Anhydrite also precipitated in the late stage of diagenesis, probably from saline Zechstein solutions [24]. The last of the authigenic minerals to crystallize was fibrous illite, at a temperature above 120 °C (e.g., [49]). Its formation is associated with the alteration of kaolinite and the recrystallization of clay minerals from the matrix [50,51,52]. Potassium for illite may have been derived from either the dissolution of potassium feldspar grains [38] or the compaction of clay rocks [51]. At a temperature of about 120 °C, blocky dickite also crystallizes [39].

5.2. Reservoir Properties

The development of pore space in the Carboniferous sandstones was most affected by compaction, cementation, and diagenetic dissolution. Mechanical compaction was more intense in the sandstones containing rock fragments, feldspar grains, and clay matrix. In contrast, in sandstones dominated by the early quartz cement, the impact of compaction was less significant. Dissolution promoted the creation of a large amount of secondary pore space, especially in sandstones with significant feldspar grain content.

5.2.1. Mississippian Sandstones

The primary porosity of the Łobżonka Shale Formation sandstones, buried to the greatest depths in Western Pomerania, has been significantly reduced by diagenetic processes such as compaction and cementation (Figure 8). The sandstones are characterized by intense quartz cementation, locally by carbonate or anhydrite cementation, and by the presence of fibrous illite (Figure 4A,B). The pore space is very poorly developed; the porosity of the sandstones is about 1%. Their porosity decreases with increasing depth (Figure 8). Microfractures or micropores tend to occur sporadically and are usually not connected to each other as the permeability of these rocks is zero (Table 4). These sandstones have unfavorable petrophysical properties as reservoir rocks.

The primary porosity of the Gozd Arkose Formation sandstones was affected mainly by compaction, cementation, and dissolution. According to Biernacka [24], arkosic arenites lost an average of 80% of their primary porosity as a result of mechanical compaction. Cements, mainly quartz and feldspar overgrowths, carbonates (Fe/Mn-calcite, Fe-dolomite), and sulfates to a lesser extent reduced the porosity and permeability by crystallizing in the pore space. The occurrence of ferruginous rims on the grain components could limit the development of the overgrowths. The formation of fibrous illite in the final stage of the cementation process caused the reduction in residual porosity. The replacement of feldspar grains and rock fragments by carbonates or sulfates also contributed to the reduction in porosity. The porosity increased due to the dissolution of mainly feldspar grains, as well as of clasts of volcanic rocks and micas, carbonate grain components, and cements, which resulted in the formation of secondary porosity (Figure 4F). The sandstones commonly show the effects of alteration processes: kaolinization, chloritization and illitization of feldspars, and chloritization and argillization of volcanic rock clasts. The alteration of mica into illite and kaolinite also has a variable impact on the porosity of the rock. The porosity of the sandstones is usually <10% (Table 4), and the porosity values commonly decrease with increasing depth (Figure 8). The microporous nature of the pore space and the irregular shapes of the pores mean that rock samples with a lower measured porosity are impermeable [17]. Locally, at higher porosity values, permeability reaches several mD. The sandstones of this formation are characterized by poor filtration properties [5].

The porosity of the Drzewiany Sandstone Formation sandstones was affected by compaction and cementation and to a lesser extent by dissolution. The degree of cementation is related mainly to silicification. With the increasing burial depth of the sandstones, the amount of quartz cement increases, which is reflected in a decrease in porosity and permeability [23]. In addition to the common quartz overgrowths on quartz grains, carbonate and sulfate cements are locally present. A characteristic feature of the sandstones is the preservation of their primary porosity (Figure 4G and Figure 5A,B). Secondary porosity, resulting from the dissolution of feldspar grains, lithoclasts, and cements, accounts for a small percentage. Most of the sandstones are characterized by very good porosity, averaging around 16%, and permeability in the range of several tens or hundreds of mD (Table 4). The high transport capacity of solutions is related to their excellent pore space parameters: high values of both the pore diameter and the average capillary, as well as to a very low hysteresis effect [18]. The average pore diameter ranges from 0.81 to 4.61 µm, which indicates the presence of macropores [13]. The small number of capillary traps means that almost the entire pore space could participate in the flow of solutions. These sandstones have very good and excellent reservoir and filtration properties [17].

Studies of the Mississippian rocks indicate the presence of sandstones with very good and good porosity (10%–30%) in the Drzewiany Sandstone Formation. Non-economic sandstones, with the porosity < 5% [53], occur in the Łobżonka Shale Formation and constitute the majority of the Gozd Arkose Formation (Table 4). The permeability of the samples from the Drzewiany Sandstone Formation ranges from 24 to approximately 1092 mD. According to Levorsen’s classification [54], they can be categorized as sandstones with very good and good permeability. On the other hand, the sandstones of the Łobżonka Shale and Gozd Arkose formations are impermeable. Measurements of porosity and permeability, as well as porosimetric data, including the number of pores >1 µm, threshold diameter, and hysteresis, indicate that the Drzewiany Sandstone Formation sandstones have the best reservoir qualities.

5.2.2. Pennsylvanian Sandstones

Porosity and permeability of the sandstones of the Wolin, Rega, and Dziwna formations were most affected by compaction, cementation, and local dissolution. Compaction reduced the primary porosity by an average of about 60% and cementation by about 40% [19,20], as calculated using the diagram of Houseknecht [55] and Ehrenberg [56] The main components of their cements are authigenic quartz, clay minerals (illite, kaolinite, dickite), and hematite, locally carbonates. The sandstones are dominated by primary porosity, while secondary porosity (dissolution of grains and cements, microporosity between crystals of authigenic clay minerals and microfractures) (Figure 5C,E,G) accounts for a negligible percentage. The sandstones are characterized by a porosity of about 10%. With the rock porosity > 10%, their permeability is usually from a few to hundreds of mD (Table 4). Permeability of the sandstones of the Wolin and Rega formations is greater than that of the Dziwna Formation sandstones, which are often impermeable. This is due to the poorer filtration properties of the Dziwna Formation sandstones, i.e., fewer pores > 1 µm, smaller threshold diameter, and higher hysteresis. The sandstones of the Wolin and Rega formations show good reservoir qualities, which are better than those of the Dziwna Formation sandstones.

The research shows that most of the analyzed Pennsylvanian sandstones have good porosity of about 10%, and only a minority with porosity < 5% are of no economic significance [53]. The samples tested range from impermeable to permeable at approximately 231 mD (Table 4). According to Levorsen’s classification [54] the rocks can be divided into sandstones with very good permeability (Wolin Formation), good permeability (Wolin and Rega formations), and satisfactory permeability (Wolin, Rega, and Dziwna formations). Based on the measurements of porosity and permeability and porosimetric data, including the number of pores > 1 µm, threshold diameter, and hysteresis, it can be concluded that the Wolin and Rega formations have the best reservoir qualities.

6. Conclusions

• The Mississippian sandstones are represented mainly by arkosic and lithic arenites and wackes of the Gozd Arkose Formation, as well as quartz arenites and wackes of the Drzewiany Sandstone and Łobżonka Shale formations. The rocks differ in terms of their grain composition and the type and content of their cementing materials. In the sandstones of the Gozd Arkose Formation, the grain framework is composed of feldspars, quartz, and lithoclasts, mostly of volcanic origin. The cementing materials are represented by a matrix and cements, the most important of which are carbonate and anhydrite. In the Drzewiany Sandstone and Łobżonka Shale sandstones, the main grain component is quartz, bound by quartz cement and matrix and locally by carbonates and anhydrite.
• The Drzewiany Sandstone Formation sandstones, among the Mississippian sandstones, are excellent reservoir rocks for hydrocarbon accumulation thanks to their mineralogical and textural maturity. Their primary porosity accounts for a significant portion of the rock’s total porosity. With increasing depth, their reservoir properties deteriorate due to increasing silicification in the sandstones. The average porosity of the sandstones is about 18%, with a permeability of even over 1000 mD. The best reservoir properties are found in quartz arenites cemented with authigenic quartz in the form of very thin overgrowths on quartz grains. The Gozd Arkose Formation sandstones may be prospective, but due to their very complex diagenetic history, their petrophysical properties are not very favorable. These rocks exhibit mainly secondary porosity (dissolution of feldspar grains and lithoclasts). They are characterized by a porosity of typically <5%, locally around 10%, and are usually impermeable. The Łobżonka Shale Formation sandstones show unfavorable reservoir properties. They occur at the greatest depths within the studied section of the Mississippian sandstones. These are massive, strongly diagenetically altered rocks. They are characterized by a porosity of about 1% and are impermeable.
• The Pennsylvanian sandstones of the Wolin, Rega, and Dziwna formations are represented by arenites and quartz wackes, locally subarcosic. The main component of the grain framework is quartz, with much smaller amounts of mica, feldspars, and rock clasts. The main components of the cementing materials are a matrix and cements: quartz, kaolinite, carbonate, anhydrite, and hematite.
• The sandstones of the Wolin and Rega formations are characterized by good reservoir properties, while the Dziwna Formation sandstones have poorer properties. The porosity of the Pennsylvanian sandstones ranges from <1 to >20%. Primary porosity dominates, while secondary porosity (dissolution of grains and cements, microcrystalline) accounts for a negligible percentage. The permeability of the sandstones of the Wolin and Rega formations exceeds 200 mD in places, most often amounting to several tens of mD. The Dziwna Formation sandstones are often impermeable. In the Pennsylvanian sections, quartz arenites, whose main cementing components are authigenic quartz and kaolinite, exhibit the best reservoir qualities.