Mechanism and Distribution of Calcareous Interbeds in Songtao Uplift and Its Periphery of Qiongdongnan Basin

Based on core and casting slice observation, well drilling and logging data, the source of carbonate materials, lithology together with electric properties, types, genesis mechanism and distribution of calcareous interbeds in Songtao uplift and its periphery of Qiongdongnan Basin have been thoroughly analyzed. Results show that typical features have been appeared from well logging curves, containing low gamma-ray, low acoustic travel time, low neutron value, high density, as well as bright white calcium nodules or bands in electrical imaging well-logging curves. Drilling results reveal that calcareous interbeds developed mostly in high position of paleostructures and their distribution was controlled by the combined effects of macroscopic and microscopic factors. Macroscopically, calcareous interbeds relate to paleogeomorphology together with the combination of sandstone and mudstone. They are also controlled microscopically by the source of carbonate cements and pore space. Under normal circumstances, with regard to the same sand, the closer to the mudstone and the thicker of mudstone, the more conducive to the formation of calcareous interlayer. Low compaction strength, high content of rigid particles, coarse grain size, well-sorted sandstone and large pore space during carbonate cementation are favorable for the development of calcareous interbeds.


Introduction
Interlayers refer to impervious bed or low-permeability layer within the sand body ranging from a few centimeters to a few meters.The morphology and distribution of interlayers are not stable due to the change of the micro-facies or phase of sand bodies caused by transient and local water flow changes.Interlayer is divided into two types: muddy interlayer and physical interlayer.With the success of S structure, it became the first potential commercial discovery in northern depression belt of Qiongdongnan Basin, but the widespread development of physical interlayers (mainly calcareous interlayers) in the area brought difficulties to the next exploration.Calcareous interlayers refer to tight sandstone with carbonate content over 10% formed inside sand body.As a seepage barrier of the reservoir, it increases the heterogeneity of the reservoir, resulting in the thinning of the effective reservoir and the deterioration of physical properties, which restrict the later exploration and development.
At present, domestic and foreign scholars focus on the identification of calcareous interlayers, as well as its qualitative evaluation, formation conditions and so on.On the source of calcareous material, Longstaff [1] [2] [3] [4] considers that the calcareous mainly comes from dissolution of feldspar and other aluminosilicates by diatomaceous organic acids and conversion of clay minerals.Xu Bei-mei [5] believes that the enrichment of calcium is closely linked with hydration of aluminosilicate minerals during early diagenetic stages and organic acidification.Huang Si-jing [6] [7] suggests that calcium is owing to atmospheric weak acid water infiltration caused by mineral dissolution.Shan Jing-fu indicates that [8] [9] the source of calcium is not only the sedimentary period but also the supergene and diagenetic stage.Based on the previous studies and sedimentary and geochemical methods, a detailed analysis of sources, characteristics, types, genesis and distribution of calcareous interlayers are provided in this paper.Meanwhile, the summary of main controlling factors and distribution are of great significance to improve the success rate of drilling.

Geological Background
The Qiongdongnan Basin is a Cenozoic fault subsidence basin along the continental margin in northern South China Sea, which is one of the largest offshore oil-rich gas basins in China.The basin is located between 17˚00'N to 18˚50'N and 108˚51'E to 114˚41'E with an area about 60,000 km 2 .The north-south dissimilitude is obvious and it can be divided into the northern depression belt (which can be subdivided into Yabei Sag, Songxi Sag and Songdong Sag from west to east), the central uplift belt (namely Yacheng uplift-Songtao uplift belt), the central depression belt (which can be further divided into Yanan Sag, Ledong Sag, Lingshui Sag, Songnan Sag and Baodao Sag) and the southern uplift belt.[11] (Figure 2).

Databases and Methods
This study was focused on the first member of Sanya Formation in Songtao up- Based on the above basic data, this study is mainly divided into the following five aspects: Above all, based on core observation, palaeontology analysis, well  logging curve, seismic facies analysis as well as regional geological data, single-well facies, planar sedimentary facies and assemblages of sandstone and mudstone were sketched by making use of corelDRAWX6, ResForm and Gxplorer software.Secondly, based on the observation and identification of rock slices (using Laica4500p microscope and its image analysis techniques), scanning electron microscopy, constant pressure mercury injection, nuclear magnetic resonance, laser particle size, heavy minerals, whole rock analysis/clay X-ray diffraction and other data, the rock composition, structure and reservoir characteristics (reservoir space characteristics and pore throat types) were clearly identified.The importance of the combination of constant pressure mercury, nuclear magnetic resonance and capillary pressure is worth emphasizing.Among them, constant pressure mercury can reflect pore throat size together with quantity and has obvious advantages in quantitative evaluation of pore throat in low permeability reservoirs.Nuclear magnetic resonance is particularly suitable for the analysis of complex lithology, complex pore structure distribution, low porosity and low permeability reservoir.Combined with the capillary pressure curve can accurately characterize the microscopic pore throat structure characteristics of a comprehensive analysis of the parameters and its impact on reservoir physical properties.Thirdly, according to the percentage content of S, R O , T max and other parameters, the diagenetic stage were rigidly compartmentalized together with judging fluid properties; fourthly, the source of carbonate materials and the mechanism of calcareous interlayers were identified through the regional tectonic data, carbon and oxygen isotopes, water analysis.Last but not the least, characteristics, controlling factors and distribution of calcareous sandstone were summarized on account of comparison between calcareous interlayers and sandstone reservoirs in the target area.3).

Characteristics of the Calcareous Interbed
Through well logging analysis, calcareous interbeds have the following unique characteristics (Figure 4): 1) Low natural gamma (less than 80 API) and Open Journal of Marine Science

Physical Characteristics
There is a good negative correlation between the carbonate cement content and the actual porosity and permeability in the first member of the Sanya Formation in the target area (Figure 5).Calcareous cement is responsible for blocking pores and throats, the properties of calcareous interlayers tend to be significantly lower than those of other sandstone sections.Compared with other good-sorting sandstone reservoirs, calcareous interbed has smaller pore and throat radius with poorer distribution centralization and poorer reservoir properties (Figures 6-8).
When the content of carbonate cements is less than 10%, the porosity is more Figure 5. Relationship between physical properties and carbonate cement content (The functional relationship between porosity and carbonate cement content is y = 20.783e−0.018x , R 2 = 0.7198; the functional relationship between permeability and carbonate cement content is y = 361.71e−0.432x , R 2 = 0.7624.) Figure 6.Capillary pressure curve comparation between calcareous interbed and sandstone reservoirs.than 15% with over 1 × 10 −3 μm 2 permeability.When the content is more than 10%, the porosity is less than 15% and the permeability is under 1 × 10 −3 μm 2 .In addition, the latter trend of reducing physical properties is slower than the former.

Genetic Mechanism of the Calcareous Interbed
Calcareous minerals (such as Ca 2+ , Mg 2+ , 3 HCO − ) is indispensable for the for- mation of calcareous interlayers, that is, it provides with both endogenous sub-stances and exogenous environment at the same time.The endogenous materials usually include the Ca 2+ in the sedimentary water or pore water, the conversion from montmorillonite to illite in mudstone, the dissolution of carbonate minerals, the dissolution and alteration of feldspar, the corrosion of iron-rich and magnesium minerals (such as mica) and so on.Exogenous environments include atmospheric water, stratigraphic water moving along faults, clay dewatering and acidic water formed by the evolution of organic matter [12] [13] [14].
The stronger the diagenesis experienced by the rock, the lower the value of δ 13 C, and the measured δ 18 O deviates greatly from the actual sedimentation value, so it is necessary to carry out "chronological effect" correction.According to the difference between the mean values of modern ocean carbonate δ 18 O and Miocene δ 18 O, the Miocene limestone was corrected for "chronological effects", and then corrected temperature was calculated by δ 18 O.On account of the analysis of carbon and oxygen isotope, the empirical formula Z = 2.048 × (δ 13 C + 50) + 0.498 × (δ 18 O + 50) [15] [16] [17] shows that the Z value of the target area is less than 120, namely the pore fluid that forms carbonate cements is related to freshwater sedimentary environment, indicating that the cements are mainly carbonate related to organic components during diagenesis (Table 1).
The data show that there are mainly three water types (NaHCO 3 , MgCl 2 and Na 2 SO 4 ) all related to the deep faults [18], resulting in CO 2 getting into the formation water (Table 2).CO 2 makes the following reaction carry on continuous- CO − (3).The combination of well (mean 0.68%) is lower than that of the S-1Sa well (mean 1.63%), and the thickness of the former calcareous interlayers (3.78 m) is also lower than the latter (17.6 m), which is consistent with the above analysis [19].
Generally speaking, carbonate cements are divided into three genetic types.
The meaning of I, II, III are diagenetic carbonate, biogas-related carbonate, and decarboxylation of organic acid associated carbonate representatively.There are two genetic types of calcareous interbed in target area: Type I and type III [20] Table 1.Carbon and oxygen isotopes and temperature statistics.Therefore, various sources of material for carbonate cements and continuous alternating of porous water are essential requirements to the continuation of the reaction.

Discussion
Since the strong randomness of calcareous interbed distribution, it's affected by the combination of multiple factors, mainly divided into macro and micro pers-    According to the stratigraphic thickness figure and drilling analysis, calcareous interbeds mostly develop in the paleo-tectonic highs (Figure 13).In view of the above, it is speculated that calcareous interbeds in Songtao uplift and its periphery mainly distributed on the margin of the sand body with high paleo-tectonics.Therefore, searching for large-scale and high-quality natural gas around without calcareous intercalation in the periphery of S structure will lay a solid foundation for expanding the scale of reserves.

Conclusions
1) The lithology of calcareous interlayers in the target area is mainly gray-white, gray-green calcareous medium sandstone and fine sandstone.Calcite, iron dolomite and dolomite are common carbonate cements.Thickness varies from tens of centimeters to several meters, making bubbles in case of acid.
2) The properties of calcareous interlayers in the first member of Sanya formation are poor with low porosity and permeability, taking the form of small pore and throat radius, low degree of their distribution.
3) The electrical characteristics are obvious with low gamma, low acoustic contrast, low neutron, high density together with bright white calcareous nodules or bands in electrical imaging logging.
4) There are two genetic types of calcareous interbed (Type I and Type III).
Among them, Type I is formed by the integration of Ca 2+ with other particles (such as feldspar).
5) There are four common types of calcareous interbeds as follows: the bottom type, the top type, the central type and the complete type.In general, the bottom type calcareous interbed develops with large thickness, while others widely distribute with low thickness.
6) Calcareous interbed is controlled by both macro and micro aspects.Macroscopically, the distribution is closely related to Paleo-tectonics and sedimentary microfacies.Microscopically, the distribution is affected by the source of material and pore space during carbonate cementation.
7) Based on the evidence presented in this research, Calcareous interlayers mostly develop in high palaeo-structures.Meanwhile, searching for large-scale traps without calcareous interlayers could improve drilling success.
Basin is located in Southeast Asia and southwestern China, prevailing in the southwest humid monsoon.It is very beneficial to the growth of terrestrial ferns, spores and herbaceous higher plants in the early Oligocene.The tectonic evolution of this basin has already undergone the Paleogene rifting stage, the early post crack with thermal-deposition stage and the late post crack P. Y. Zhu et al.DOI: 10.4236/ojms.2018.83020372 Open Journal of Marine Science as well as accelerated sedimentation stage during Neogene [10] (Figure 1

Figure 1 .
Figure 1.(a) Schematic configuration of the Qiongdongnan Basin; (b) The sub-tectonic units of the study area and well locations.

Figure 7 .
Figure 7. Pore and throat comparation between calcareous interbed and sandstone reservoirs in constant pressure mercury.

Figure 8 .
Figure 8. Pore and throat comparation between calcareous interbed and sandstone reservoirs in nuclear magnetic resonance.

3 HCO
− ) and Ca 2+ (Mg 2+ ) contribute to the formation of carbonate Cement.For example, the CO 2 content of the S-1

Figure 9 . 3 CO
Figure 9. Carbon and oxygen isotopes characteristics of carbonate cements.
pective.P. Y. Zhu et al.DOI: 10.4236/ojms.2018.83020380 Open Journal of Marine Science Macroscopically, the calcareous intercalations in the target area have poor lateral continuity, large thickness variation and closely related to sedimentary microfacies.However, the distribution of calcareous interbeds in different parts of the same genetic sand body varies with types of rock combination.There are four common types of calcareous interbeds as follows: the bottom type, the top type, the central type and the complete type [25] [26] [27] [28] [29].Among them, the bottom type is mostly formed in the lower part of positive cyclic sandbody.The top type developed on the top of the anti-cyclic sandbody or distal bar, when the deposition is relatively stable.The central type calcareous interbed can be seen in underwater distributary channel or estuary dam owing to sand bodies vertically superimposed.The complete type calcareous interlayer isthose isolated thin sand layer wrapped in mudstone, the entire sand was completely cemented by carbonate (Figure10).In general, the bottom type calcareous interbed develops with large thickness, while others widely distribute with low thickness.Microscopically, the distribution of calcareous interbed is controlled by the source of material and pore space during carbonate cementation.The former is influenced by mudstone thickness and the distance between sandstone and mudstone[30] [31][32].For the same sand layer, the greater thickness of mudstone, the closer to the sand body, the more abundant of the source, the more favorable to the formation of the calcareous interlayer.The latter is closely related to the petrological characteristics of the sand body and later diagenetic evolution.The high rigid content, coarse grain size and well-sorted sandstones which have good primary porosity and strong anti-compaction ability are prone to the formation of calcareous interlayers (Figure11& Figure12).

Figure 11 .
Figure 11.The controlling factor analysis of calcareous interbed.

4.1.1. Lithology and Electric Properties
are commonly found in the target area (Figure

Table 2 .
Water analysis statistics of the target area.