Porosity distribution in cyclic dolomites of the Lower Qiulitag Group (Upper Cambrian) in northwestern Tarim Basin, China

Location of the study area and general structural units in the Tarim Basin (modified from Lin CS et al., 2011). 1−Yong’anba section (YAB); 2−Keping section (KP); 3−Tonggusibulong section (TGL); 4−Penglaiba section (PLB).

Stratigraphic system of Cambrian in the northwestern Tarim Basin, mainly based on the data from references (Zhou ZY et al., 1990; Zhang SB and Gao QQ, 1992; Zhou LK et al., 1991; Zhou ZY, 2001). Sea-level curve from the publications by Haq BU and Schutter SR, 2008, Zhang YQ et al., 2015.

Schematic facies model of a carbonate ramp system in the study area (modified from Zhang YQ et al., 2015). A−E−show the meter-scale depositional successions (or cycles) in different environments, from supratidal, to intertidal, to restricted shallow subtidal, and shoals and intermediate subtidal facies. The high porosity is present in stromatolite (L4), thrombolite (L5), cross-bedded packstone/grainstone lentil (L6) and microbial buildup (L9), while low porosity is generally present in thin laminites (L1), wavy laminites (L3), ribbon dolomites (L7), oolites (L8) and lenticular to platy dolomites (L10). LTL−low tidal level; HTL−high tidal level.

Intertidal to restricted shallow subtidal facies. a–thick laminites (L2) with centimeter-scale laminae, TGL section. b–photomicrograph of thick laminites (L2) showing alternations of dark interlocking nonplanar dolomites and light finely planar dolomites. Abundant intercrystalline pores (in blue) are observed in PLB section. Planar-polarized light. c–domical stromatolite showing smooth to convoluted individual laminae (L4), PLB section. Scale in centimeters. d–photomicrograph showing stromatolite lamina with dark cryptmicrobial and light-colored, porous micropeloid couplets (pores are shown in blue), PLB section. Planar-polarized light. e–stratiform thrombolites (L5) with dendroid to digitate mesoclots, PLB section (Zhang YQ et al., 2015). Scale in centimeters. f–photomicrograph of thrombolite (L5) showing dark-gray irregular microbial micrite clots (or thromboses) and dissolution vugs (in blue), PLB section. Planar-polarized light. Blue areas are epoxy-impregnated porosity.

Porosity distribution in cycles. a–peritidal cycle with a thrombolite base (A) and a stromatolite cap (B). Note the cap is more porous than the base of the cycle, PLB section. Hammer for scale (30 cm). b–photomicrograph of stromatolite (B in Fig. 5a). Note pores in red color. Planar-polarized light. c–photomicrograph of thrombolite (A in Fig. 5a). Note vugs in red color. Planar-polarized light. d–a typical shallow subtidal cycle with thin-bedded lenticular peloidal packstone/grainstone (L10, A) base, thick-bedded peloidal grainstone middle (B), and thrombolitic mounds (C, within dashed lines) cap, TGL section (Zhang YQ et al., 2015). Porosity increases upwards in a cycle. The standing person for scale (165 cm). e–photomicrograph of the thrombolite mound (C in Fig. 5d) showing small vugs (in blue) partly filled with planar dolomite cement (dc), TGL section. Planar-polarized light. f–photomicrograph of the thick ooidal grainstone (B in Fig. 5d) showing interparticle pores (in blue) and ghost ooids with quartz (Q) infills, TGL section. Planar-polarized light. g–photomicrograph of the thin peloidal grainstone (A in Fig. 5d) showing tightly compacted planar/nonplanar dolomite crystals with minor pores, TGL section. Planar-polarized light. Blue/red areas are epoxy-impregnated porosity.

Shallow subtidal facies. a–cross-bedded (dashed lines) pebbly peloidal-ooidal grainstone (L8) with multiple internal erosive surfaces (arrows), KP section(Zhang YQ et al., 2015). Scale in centimeters. b–pebbly peloidal-ooidal grainstone (L8). Note the vague internal fabrics of ooids are only with their ghosts and micrited peloids in pores (in blue), TGL section. Planar-polarized light. c–isolated, columnar microbial (thrombolite) buildups (L9, with circles) in the upper part of the Lower Qiulitag Group. Inter heads are filled with peloidal-ooidal grainstones (light-colored), TGL section. Standing person (left) for scale (about 0.7m). The inset photo shows the internal fabrics of the microbialite (Hammer is 30 cm). d–e– photomicrographs showing the microbial buildups (L9). Note vuggy and inter crystal porosity (in blue) and micro-clots (dark-brown color). Dolomite cement (dc) fills along pores, PLB section. Planar-polarized light. f–photomicrograph of peloidal grainstones (L11) showing micrited peloids mostly cemented by later dolomite crystals (white-gray color) and small pores (in blue). Note the presence of crinoid fragments (arrow). TGL section. Planar-polarized light.

Porosity distribution in meter-scale cycles and sequences of the Lower Qiulitag Group from the four measured sections (see Fig. 1 for the location; modified from Zhang YQ et al., 2015). Sequence stacking patterns reveal a synoptic accommodation (or sea-level) decrease from Sq1 to Sq3 and an increase from Sq4 to Sq6, defining two sequence sets. Abundant porosity is generally present in peritidal cycles of regressive successions and in thick shallow subtidal cycles of transgressive successions. Porosity-measured cycles are marked by shade. Porosity $ \geqslant$4% is marked on the side of cycle curves.

Porosity distributions in the third-order transgressive-regressive succesions of the Lower Qiulitag Group at the PLB section. High porosity is widely present in the thick thrombolites and thick-laminites in the transgressive intervals. The porosity increases upwards in cycles in the lower part, while decreases upwards in cycles in the upper part of the regressive succession of Sq3. Abundant porosity occurs around sequence boundaries. SB−Sequence boundary. Short lines on the right of logs mark the cycle boundaries. See Fig. 3 for the legend.

Porosity distribution in the third-order sequence Sq6 of the Lower Qiulitag Group. Abundant porosity generally occurs in the thick subtidal cycles during the transgressive stage. The porosity commonly shows an upward-decreasing trend in the transgressive intervals. The porosity is extremely variable around sequence boundaries (between Sq5 and Sq6). A–PLB section; B–KP section; C–YAB section. SB−Sequence boundary. Short lines on the right of logs mark the cycle boundaries. See Fig. 3 for the legend.

Photomicrographs showing vuggy pore in grainstones below the sequence boundaries. Planar-polarized light. a–vugs (in blue) in the peloidal-ooidal grainstones. Note vugs are largly unfilled. PLB section. b–vugs in planar matrix dolomites. Note vugs are filled by large calcite crystal and dolomite cements (dc). Floating planar dolomites are present on the surfaces of calcite crystals, KP section.

The distribution of porosity in lithofacies of the Lower Qiulitag Group. The high porosities (>4.0%) are present in L2, L4, L5, L6 and L9.

Photomicrographs of dolomite types and related porosity. Planar-polarized light. a–micritic dolomite, PLB section. b–planar dolomite, TGL section. c–planar dolomite with vuggy and intercrystalline porosity (in blue), PLB section. d–planar and nonplanar dolomites separated by stylolite, KP section. e–the vug (in red) partly filled with bitumen (Bt) and quartz (Q), PLB section. f–saddle dolomites showing curved crystal face along the pores. Small pores are preserved in the center of pore spaces (in blue), KP section.