Mechanism of carbonate cementation and its influence on reservoir in Pinghu Formation of Xihu Sag
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Abstract: Carbonate cements are the most abundant authigenic mineral and impact on physical properties greatly in sandstone reservoir. In this paper, Pinghu Formation of Xihu Sag was taken as a target. Characteristics, distribution and formation of carbonate cements were investigated via optical microscopy, cathodoluminescence (CL), electron probe and in-situ carbon-oxygen isotope. The results showed that carbonate cements varied in types and shapes. Calcite/dolomite mainly present as poikilotopic cements, while ferrocalcite/ferrodolomite/ankerite generally present as pore-filling cements. Carbon isotope (δ13C) values of carbonate cements were ranging from –7.77‰ to –2.67‰, with an average of –4.52‰, while oxygen isotope (δ18O) values were ranging from –18.94‰ to –12.04‰, with an average of –14.86‰. The δ13C/δ18O indicated that the paleo-fluid of carbonate cement was mainly freshwater. Organic carbon mainly came from organic matter within mature source rocks, and inorganic carbon came from dissolution of carbonate debris and early carbonate cements. Distinctive δ13C/δ18O values manifest that carbonate cements with different types formed in different periods, which make different contributions to the reservoir properties. Calcite/dolomite formed during eodiagenesis (70–90℃) and early mesodiagenesis stage (90–120℃), and were favorable to reservoir owing to their compacted resistance and selective dissolution. Ferrocalcite/ferrodolomite/ankerite formed during middle-late mesodiagenetic stage (above 120℃), and were unfavorable to reservoir due to cementing the residual intergranular pores. Hence, in order to evaluate the reservoir characteristics, it is of significantly important to distinguish different types of carbonate cements and explore their origins.
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Key words:
- carbonate cements /
- genesis mechanism /
- Xihu Sag /
- Pinghu Formation
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Figure 1. Tectonic belt in Xihu Sag and stratigraphic sequence in the study area. a is modified after Shanghai Company of China National Offshore Oil Corporation (CNOOC), and the red rectangle in a is the study area. b is the comprehensive column map from of the study area. Mbr.: member; GR: gamma Ray; API: American Petroleum Institute; Lith.: lithology; HST: highstand system tract; TST: transgressive system tract; LST: lowstand system tract.
Figure 3. Characteristics of carbonate cements in Pinghu Formation. a. W1, 3439.47 m, p3, ferrocalcite terrestrial carbonate fragment or intraclast limestone (yellow arrow) (plane-polarized light); b. W2, 4 297.1 m, p1, cathodoluminescence feature of pore filling cementation calcite (yellow arrow) (CL); c. W3, 4 206.77 m, p5, cathodoluminescence feature of poikilotopic cementation Calcite (yellow arrow) (CL); d. W1, 3439.57 m, p3, ferrodolomite filling residual intergranular pores (yellow arrow) (plane-polarized light); e. W4, 4019.9 m, p2, cathodoluminescence feature of grain-embedded ferrocalcite (yellow arrow) (CL); f. W4, 4202.87 m, p3, dolomite filled intergranular pore(yellow arrow) (CL); g. W5, 4086.9 m, p2, calcite and ferrocalcite staining characteristics in siltstone (yellow arrow) (plane-polarized light); h. W6, 4247 m, p3, pore filling cementation ferrocalcite, ferrodolomite filling intergranular pores or terrestrial carbonate fragment or intraclast limestone (yellow arrows) (plane-polarized light); i. W7, 3754 m, p3, calcite replaced by ferrocalcite, ferrodolomite replaced by ferrocalcite (plane-polarized light). An: ankerite; Cc: calcite; Do: dolomite; CL: cathode luminescence.
Figure 4. Trace elements and luminescence characteristics of calcite. a. Fe2+, Mn2+ and Fe2+/Mn2+ of calcite, and the abscissa is the sample number; b. W7, 4406.85 m, p2, cathodoluminescence image of calcite (CL); c. W7, 4394.11 m, p2, cathodoluminescence image of ferrocalcite (CL).
Figure 5. CaO-FeO-MgO classification of carbonate cements (I: calcite, II: Fe calcite, III: dolomite, IV: Fe dolomite, V: ankerite, adapted from Liu et al., 2011).
Figure 6. Relationship between grain size and cements (calcite, dolomite). a. Relationship between sandstone grain size and calcite content; b. relationship between sandstone grain size and dolomite content. Red dashed line represents trend line.
Figure 7. Distribution characteristics of carbonate cements. a. Distribution of carbonate cements in different layers of Pinghu Formation with depth; b. W1, 3 444.67 m, p3, granular ferrocalcite replaced debris particles (plane-polarized light); c. W1, 3447.27 m, p3, poikilotopic cementation ferrocalcite (plane-polarized light); d. W4, 4202.77 m, p3, poikilotopic cementation ferrocalcite (plane-polarized light). The locations of sampling points of b, c and d are shown in a.
Figure 8. Genesis type diagram of carbonate cement in Pinghu Formation (modified after Wang and Zhao, 2001).
Figure 9. Temperature dependence of dissolution porosity, calcite, Fe2+, Mg2+ and Fe2+/Mn2+.
Figure 10. Relationship between carbonate cements and physical properties. a. The cross plot diagram of (Fe) calcite content and porosity; b. the cross plot diagram of (Fe) calcite content and permeability; c. the cross plot diagram of (Fe) dolomite/ankerite content and porosity; d. the cross plot diagram of (Fe) dolomite/ankerite content and permeability.
Table 1. Results of carbon and oxygen isotope in carbonate cement of Pinghu Formation
Well name Member Depth/m Carbonate minerals $\text{δ} $13CPDB/‰ $\text{δ} $18OPDB/‰ Z Diagenesis temperature/℃ T1 T2 T3 Average W8 p5 4339.23 calcite –4.16 –13.67 112.0 77.8 112.5 173.9 121.4 W8 p5 4342.89 calcite –3.96 –12.64 112.9 57.2 83.4 118.7 86.4 W8 p5 4340.89 calcite –3.58 –12.87 113.6 58.7 85.1 120.7 88.2 W8 p5 4342.89 ferrocalcite –5.16 –18.59 107.5 100.0 132.5 175.1 135.9 W8 p5 4340.89 ferrocalcite –4.62 –18.75 108.5 101.2 133.9 176.7 137.3 W8 p5 4342.89 dolomite –4.25 –13.56 111.8 76.9 111.5 172.7 120.4 W3 p3 4219.26 calcite –4.65 –12.21 111.7 54.6 80.3 115.1 83.3 W3 p3 4196.05 calcite –4.06 –12.61 112.7 57.0 83.2 118.5 86.2 W3 p3 4196.55 calcite –3.95 –12.57 113.0 56.8 82.8 118.1 85.9 W9 p1–p2 4344.79 calcite –6.25 –14.94 107.1 88.8 125.2 188.5 134.2 W9 p1–p2 4345.59 calcite –5.85 –14.06 108.3 66.5 94.2 131.2 97.3 W9 p1–p2 4348.69 calcite –5.02 –13.11 110.5 60.2 86.9 122.8 90.0 W9 p1–p2 4348.69 calcite –4.96 –15.06 109.6 73.4 102.1 140.5 105.3 W9 p1–p2 4344.79 calcite –4.95 –13.57 110.4 76.9 111.5 172.8 120.4 W9 p1–p2 4348.69 calcite –4.91 –13.27 110.6 74.4 108.7 169.4 117.5 W9 p1–p2 4345.09 calcite –3.41 –12.68 114.0 57.5 83.7 119.1 86.8 W9 p1–p2 4348.69 ferrocalcite –6.76 –14.30 106.3 68.2 96.1 133.5 99.2 W9 p1–p2 4344.79 ferrocalcite –5.64 –17.11 107.2 109.0 148.2 214.7 157.3 W9 p1–p2 4348.69 ferrocalcite –4.35 –18.34 109.3 97.9 130.1 172.5 133.5 W10 p1–p2 4471.2 ankerite –4.96 –18.42 108.0 122.0 162.9 231.3 172.1 W7 p1–p2 4397.54 calcite –3.87 –14.01 112.4 66.2 93.8 130.8 97.0 W7 p1–p2 4394.11 calcite –2.67 –12.67 115.5 57.4 83.6 119.0 86.7 W7 p1–p2 4406.85 ferrocalcite –4.42 –18.41 109.1 98.5 130.8 173.2 134.2 W7 p1–p2 4397.54 ferrocalcite –3.95 –18.82 109.8 101.8 134.5 177.4 137.9 W7 p1–p2 4394.51 ferrodolomite –5.98 –17.56 106.3 113.4 153.2 220.3 162.3 W7 p1–p2 4394.51 ferrodolomite –3.30 –17.53 111.8 113.1 152.9 220.0 162.0 W11 p5 4565.43 ferrocalcite –5.42 –16.21 108.1 81.7 111.7 151.4 114.9 W11 p5 4565.43 ferrocalcite –4.97 –15.42 109.4 76.0 105.1 143.8 108.3 W1 p3 3439.87 calcite –2.69 –12.81 115.4 58.3 84.6 120.2 87.7 W1 p3 3447.38 dolomite –4.08 –12.79 112.6 70.5 104.1 164.2 112.9 W1 p3 3447.38 dolomite –3.81 –13.07 113.0 72.7 106.7 167.2 115.5 W1 p3 3447.07 dolomite –3.51 –12.54 113.9 68.4 101.7 161.4 110.5 W1 p3 3447.38 dolomite –3.40 –12.04 114.3 64.4 97.0 156.0 105.8 W1 p3 3447.38 dolomite –3.08 –13.67 114.2 77.8 112.6 173.9 121.4 W1 p3 3443.07 ankerite –5.28 –17.90 107.6 116.8 157.0 224.7 166.2 W1 p3 3439.47 ankerite –4.08 –17.70 110.1 92.9 124.4 166.0 127.8 W5 p2–p3 4085.85 calcite –4.84 –12.40 111.2 55.7 81.6 116.6 84.6 W5 p2–p3 4093.45 dolomite –4.67 –13.91 110.8 79.9 114.9 176.6 123.8 W5 p2–p3 4084.28 dolomite –4.20 –13.26 112.1 74.4 108.6 169.3 117.4 W5 p2–p3 4085.26 dolomite –3.92 –14.18 112.2 67.3 95.1 132.3 98.2 W5 p2–p3 4086.56 ferrodolomite –7.77 –15.90 103.5 79.4 109.0 148.4 112.3 W5 p2–p3 4084.28 ankerite –4.68 –18.94 108.3 127.3 168.9 238.1 178.1 Note: T1 is the temperature when the fluid is meteoric water ($\text{δ}^{18} {\rm{O}}_{{\rm{Water}}}({\rm{SMOW}}) $=–5‰), T2 is the temperature when the fluid is pleistocene ice age water ($\text{δ}^{18} {\rm{O}}_{{\rm{Water}}}({\rm{SMOW}}) $=–1.2‰), and T3 is the temperature at which the fluid is formation water ($\text{δ}^{18} {\rm{O}}_{{\rm{Water}}}({\rm{SMOW}}) $=2‰). 
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