The influence of bioturbation on sandy reservoirs: the delta front sand of the lower Zhujiang Formation, Baiyun Depression, Zhujiang River Mouth Basin
Abstract: Ichnofossils are well developed in clastic rock reservoirs in marine and transitional facies, which can considerably change the physical properties of the reservoir. However, this influence is not well understood, raising an important problem in the effective development of petroleum reservoirs. This paper analyzes continental shelf margin delta reservoirs through core observation, cast thin section observation and reservoir physical property test. Some important scientific insights are obtained: (1) The presence of Cruziana ichnofacies, including Asterosoma, Ophiomorpha, Planolites, Skolithos, Thalassinoides, and other ichnofossils can be used to identify in subaqueous distributary channels, subaqueous levee, frontal sheet sand, abandoned river channels, crevasse channels, main channels and channel mouth bars. Considerable differences in the types of ichnofossils and the degree of bioturbation can be observed in the different petrofacies. (2) Ichnofossils and bioturbation play a complex role in controlling reservoir properties. The reservoir physical properties have the characteristics of a decrease–increase–decrease curve with increasing bioturbation degree. This complex change is controlled by the sediment mixing and packing of bioturbation and the diagenetic environment controlled by the ichnofossils. (3) Sea-level cycle changes affect the modification of the reservoir through sediment packing. Bioturbation weakens the reservoir’s physical property when sea level slowly rises and improves the reservoir’s physical property when base level slowly falls.
Figure 2. Schematic summary of the depositional evolution, sequence stratigraphy framework, and main tectonic movements in the Zhujiang River Mouth Basin during the Cenozoic period (modified from Lin et al. (2018a), SQhj3: Sequence Hangjiang Fm. 3; SQzj4: Sequence Zhujiang Fm. 4; SQzh2: Sequence Zhuhai Fm. 2.
Figure 3. Micrograph of sandstone sample. a–d. Micrograph of the sandstone sample at depth of 3 735.03 m; a. Thin section image, the sample is a medium-fine grained sandstone, where the rock is compact; b. SEM image, overall, the intergranular pores are 30–100 μm with good connectivity; c. SEM image, feldspar is altered into flaky mica (m) and solution pores (sp); d. SEM image, quartz is dissolved into dissolution pits (dp) mixed with lamellar illite/smectite (i/s) layer. e–h. Micrograph of the sandstone sample at depth of 3 742.28 m; e. Thin section image, the sample is a medium-grained sandstone, where the rock is compact; f. SEM image, overall, the intergranular pores are 20–60 μm with good connectivity; g. SEM image, intergranular lamellar illite/smectite (i/s) mixed layer and kaolinite (k); h. SEM image, intergranular flaky illite (i), kaolinite (k), quartz overgrow (qo) to Level III.
Figure 6. Sedimentary characteristics of petrofacies. a–d. Sedimentary characteristics of subaqueous distributary channel. a. 3 735.33–3 735.50 m, fine-, medium-, to coarse-grained sandstone with tabular cross-bedding in the upper part. Conichnus (Co), Ophiomorpha (Op), and escape trace (e.t.) are contained near the scour surface. b. 3 742.83–3 743.00 m, medium- to coarse-grained massive sandstone with Thalassinoides (Th). c. 3 746.00–3 746.16 m, medium- to coarse-grained sandstone with massive bedding with Diplocraterion (Di), Ophiomorpha (Op), Skolithos (Sk), and Thalassinoides (Th). d. 3 750.31–3 750.45 m, fine-, medium- to coarse-grained sandstone, with parallel bedding in the upper part and small cross-bedding in the lower part. e–h. Sedimentary characteristics of the channel mouth bar. e. 3 732.86–3 733.04 m, fine- to medium-grained sandstone with fine gravel with Ophiomorpha (Op), Palaeophycus (Pa), andThalassinoides (Th). f. 3 745.54–3 745.68 m, fine-grained sandstone at the bottom, muddy stripe thereon, and scour surface at the top. g. 3 746.81–3 746.93 m, fine- to medium-grained sandstone with Ophiomorpha (Op), Palaeophycus (Pa), Rhizocorallium (Rh), and Skolithos (Sk). h. 3 746.99–3 747.20 m, fine-grained sandstone with Skolithos (Sk) and Thalassinoides (Th) and inverse graded bedding. i. Sedimentary characteristics of the abandoned channel and crevasse channel, 3 737.39–3 737.63 m; the bottom is gray coarse-grained sandstone with fine gravel of the crevasse channel, with discontinuous muddy lamination and escape trace (e.t.). Above it is the development of dark gray mudstone with Planolites (Pl) and unidentified dwelling burrows (dw) filled with medium- to coarse-grained sand. The top is a thin layer of silty sandstone in the abandoned channel with ripple cross lamination and Thalassinoides (Th) filled with medium-grained sand. j. Sedimentary characteristics of the frontal sheet sand, 3 739.30–3 739.45 m, fine- to medium-grained sandstone with inverse graded bedding and Ophiomorpha (Op). k. Frontal sheet sand, 3 739.56–3 739.68 m, contained Ophiomorpha (Op) and Thalassinoides (Th). l. Sedimentary characteristics of the subaqueous levee, 3 753.51–3 753.67 m, the lower is siltstone and fine-grained sandstone with muddy laminae and Ophiomorpha (Op), Thalassinoides (Th), escape trace (e.t.), the upper is fine- to medium-grained sandstone.
Figure 9. Relationship between bioturbation quantity and porosity. a. Relationship between bioturbation quantity and porosity in Well PY-X; b. relationship between bioturbation quantity and porosity of fine-grained sandstone samples; c. relationship between bioturbation quantity and porosity of medium-grained sandstone samples; d. relationship between bioturbation quantity and porosity of coarse-grained sandstone samples.
Figure 11. Thin-sectioned characteristics of sandstone samples with NBT or WBT. a. 3 736.03 m, medium- to fine-grained sandstone, medium sorting degree, point-line contact, contact-porous cementation type. Particles are mainly fine-grained sands, and some are medium- and very fine-grained sands. The pores are moderately developed and are distributed uniformly. The secondary is the main type, including secondary inter-granular pores (sep), secondary intra-granular pores (sap), and mold pores. The pore shape is irregular and the size is generally between 0.10 mm and 0.25 mm. The shape of the primary intergranular pores is mainly triangular and irregular, and the size is generally between 0.05 mm and 0.15 mm. Kaolinite (k) and quartz overgrow (qo) can be observed. b. 3 750.78 m, medium- to fine-grained sandstone, medium sorting degree, point-line contact, contact-porous cementation type. Particles are mainly medium-grained sand, some are coarse-grained sand and a small amount of fine-grained sand. The pores are mainly secondary pores with the inhomogeneous distribution. Their types are mainly secondary intergranular (sep) and granular dissolving pores with irregular shapes that have a general size of 0.15 mm and 0.40 mm. Primary intergranular pores (pep) are scarce. Kaolinite (k) and quartz overgrow (qo) can be observed. c. 3 756.28 m, medium- to coarse-grained sandstone, medium to a poor sorting degree, point-line contact, contact-porous cementation type. Particles are mainly medium- and coarse-grained sand. The pores are generally well developed. The pores are mainly secondary pores, including mold pores and secondary intergranular pores (sep). The pore shape is mainly irregular, and the size is generally between 0.15–0.60 mm. The shape of the primary intergranular pores (pep) is mainly triangular and irregular, and the size is generally 0.10–0.20 mm. Kaolinite (k) and quartz overgrow (qo) can be observed.
Figure 12. Thin-sectioned characteristics of sandstone samples with MBT. a. 3 735.53 m, medium- to coarse-grained sandstone, poor to medium sorting degree, point-line contact, and contact-pore cementation type. The amount of medium-grained sand particles is slightly higher than that of coarse-grained sand particles and small amounts of fine-grained sand particles also occur. The pores are moderately developed and uniformly distributed. The secondary pores, including mold pores (mp) and secondary intergranular pores (sep), represents the most common kind of pore. The shape of the secondary intergranular pores is irregular and their size is generally between 0.10–0.25 mm. Kaolinite (k) and quartz overgrow (qo) can be observed. b. 3 743.78 m, medium- to coarse-grained sandstone, poor to medium sorting degree, point-line contact, porous cementation type. The amount of medium-grained sands particles is slightly higher than that of coarse-grained sand particles, and a small amount of fine-grained sand praticles can also be observed. The pores, mainly secondary pores, are poorly developed and uniformly distributed, such as secondary intergranular pores (sep). The shape is irregular, and the size is generally between 0.10–0.30 mm. The primary pores are mainly intergranular pores with poor connectivity. Kaolinite (k) can be observed. c. 3 746.80 m, medium- to fine-grained sandstone, poor to medium sorting degree, concavo-convex contact, and porous cementation type. The amount of fine-grained sand particles is slightly higher than that of medium-grained sand particles, and a small amount of coarse-grained sand particles can also be observed. Microcracks (mc) can occasionally be observed with widths of 0.03–0.05 mm.
Figure 13. Thin-sectioned characteristics of sand samples with severe bioturbation from various ichnogenus. a. 3 732.07 m, medium- to fine-grained sandstone, poor to medium sorting degree, point-line contact, and porous cementation type. Fragment particles are mainly silty and fine-grained sands, with fewer medium-grained particles. The pores are poorly developed, distributed in homogeneously, and are mainly secondary dissolved pores with poor connectivity. Quartz overgrow (qo) can be observed. b. 3 732.53 m, medium- to fine-grained sandstone, poor sorting degree, point-line contact, and porous cementation type. Particles are contained in all grades, including fine-, medium-, and coarse-grained. Pores are poorly developed and granular and intergranular dissolved pores can occasionally be observed with poor connectivity. Kaolinite (k) and quartz overgrow (qo) can be observed. c. 3 747.66 m, fine-grained sandstone, medium to a good sorting degree, point-line contact, porous cementation type. Particles are mainly fine-grained with a small amount of medium-grained sands. The pores are poorly developed. Zircon (Zr) can occasionally be observed.
Table 1. Bioturbated classification scheme
BI Taylor and Goldring (1993) Droser and Bottjer (1986) This research scheme (modified from
Droser and Bottjer (1986))
Classification 0 0 no-bioturbation 0 non-bioturbation 0 non-bioturbation 1 1–4 sparse bioturbation 0–10 discrete, isolated trace fossils 1–20 weakly bioturbated, massive,
parallel and tabular cross bedding
distinct boundaries and low
discrete traces density
2 5–30 low bioturbation 10–40 burrows are generally isolated,
but locally overlap
21–60 moderately bioturbated, massive
and parallel bedding with sharp
boundaries and rare discrete
moderate bioturbation 40–60 burrows overlap and are not
always well defined
4 60–90 high bioturbation 60–100 bedding is completely disturbed,
but burrows are still discrete in
places and the fabric is not mixed
61–100 pervasively bioturbated, massive and inverse graded structures with indistinct to completely disturbed bedding boundaries, high trace density with common overlap to repeated overprinting 5 91–99 intense bioturbation 6 100 complete bioturbation 100 bedding is nearly or
Note: BI: bioturbation index.
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