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.
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 [
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 km2. 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. Qiongdongnan 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 as well as accelerated sedimentation stage during Neogene [
This study was focused on the first member of Sanya Formation in Songtao uplift and its periphery of Qiongdongnan Basin where most of calcareous interbeds are widely distributed. Consequently, most of the drilling for gas has taken place in this area producing core material, debris, sidewall coring, well logs and other exploration and production well data made available for this research. All these data was derived from more than 10 wells.
A rich and valuable supply of analytical laboratory data in early Miocene and late Oligocene were gained from CNOOC Energy Development and Engineering Co. Ltd., China University of Petroleum, Chengdu University, Yangtze University and Research Institute of Geology, Shengli Oilfield Co. Ltd., Sinopec, which include 334 thin section identification samples, 336 granularity analysis points, 412 reservoir porosity and permeability data points, 268 heavy mineral samples, 210 clay X-ray diffraction points, 151 rock pyrolysis samples, 53 reflectance of vitrinite measurement, 49 carbon and oxygen isotope determination, 17 nuclear magnetic resonance samples, 34 constant pressure mercury points, 26 water analysis data and 20 scanning electron microscopy observation and analysis.
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, RO, Tmax 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.
The calcareous interbed in research area is mainly gray or gray-green calcareous cemented fine sandstone and medium sandstone, which not evenly distributes generally ranging from tens of centimeters to several meters, making bubbles in case of acid. Quartz and feldspar are main components of calcareous interbed with mean value of 55.8% quartz content, average value of 14% feldspar as well as 8.5% debris content. Calcareous interlayers are characterized by moderate structure maturity, mostly point or point-line contact relation, whose interstitial materials are mainly carbonate cements (ferrocalcite gives priority, followed by ferrodolomite and dolomite), usually in the form of strips, pyramids or polygons. The main type of porous cementation and filling replacement cementation are commonly found in the target area (
Through well logging analysis, calcareous interbeds have the following unique characteristics (
no expanding diameter in the caliper curve 2) Higher lateral resistivity than any other sandstone sections 3) Obviously low interval transit time (generally below 75 μs/ft, showing a “spike” in the negative direction) 4) High density (the average value is 2.62 g/cm3) 5) Low neutron 6) Light yellow in electrical imaging logging performance with bright white calcium tuberculosis or bright white calcium band.
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 (
than 15% with over 1 × 10−3 μm2 permeability. When the content is more than 10%, the porosity is less than 15% and the permeability is under 1 × 10−3 μm2. In addition, the latter trend of reducing physical properties is slower than the former.
Calcareous minerals (such as Ca2+, Mg2+, HCO 3 − ) is indispensable for the formation of calcareous interlayers, that is, it provides with both endogenous substances and exogenous environment at the same time. The endogenous materials usually include the Ca2+ 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 [
The stronger the diagenesis experienced by the rock, the lower the value of δ13C, and the measured δ18O 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 δ18O and Miocene δ18O, the Miocene limestone was corrected for “chronological effects”, and then corrected temperature was calculated by δ18O. On account of the analysis of carbon and oxygen isotope, the empirical formula Z = 2.048 × (δ13C + 50) + 0.498 × (δ18O + 50) [
The data show that there are mainly three water types (NaHCO3, MgCl2 and Na2SO4) all related to the deep faults [
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 [
Well | Horizon | Lithology | Depth (m) | δ13CPDB | δ18OPDB | δ13CPDB calibration | δ18OPDB calibration | Temperature (˚C) | Corrected temperature (˚C) | Z value |
---|---|---|---|---|---|---|---|---|---|---|
(‰) | ||||||||||
C-1 | SY1 | Calcareous mudstone | 2824 | −19.26 | −4.23 | −20.76 | −1.63 | 35.8 | 23 | 85.7 |
Biological limestone | 2826 | −5.02 | −3.43 | −6.52 | −0.83 | 31.7 | 19.4 | 115.3 | ||
Calcareous fine sandstone | 2826 | −10.5 | −5.09 | −12 | −2.49 | 40.3 | 27.1 | 103.3 | ||
2830 | −20.39 | −1.86 | −21.89 | 0.74 | 24.1 | 12.6 | 84.6 | |||
Biological limestone | 2830 | −6.89 | −5.71 | −8.39 | −3.11 | 43.7 | 30.1 | 110.3 | ||
SY2 | Calcareous mudstone | 2862 | −9.74 | −10.02 | −11.24 | −7.42 | 69.1 | 53.3 | 102.4 | |
Calcareous fine sandstone | 3232 | −1.73 | −8.77 | −3.23 | −6.17 | 61.4 | 46.2 | 119.4 |
Well | Depth, m | Horizon | PH | K+ + Na+, mg/L | Ca2+, mg/L | Mg2+, mg/L | CO 3 2 − , mg/L | HCO 3 − , mg/L | Total minerality, mg/L | Water type | Distance of main target layer from faults, km |
---|---|---|---|---|---|---|---|---|---|---|---|
L-1 | 2677 | LS1 | 7.5 | 14257 | 58 | 0 | 33 | 940 | 37,531 | NaHCO3 | 1.8 |
B-1 | 2141 | SY1 | 6.52 | 12075 | 144 | 86 | 0 | 2441 | 32,563 | NaHCO3 | 1.1 |
Y9 | 2515 | LS3 | 7 | 4822 | 274 | 408 | 16 | 612 | 15,055 | MgCl2 | 0.28 |
Y9 | 2508 | LS3 | 6.4 | 110.19 | 548 | 1150.81 | 0 | 169 | 34,727 | MgCl2 | |
Y9 | 2509 | LS3 | 6.4 | 11090 | 548 | 1147.89 | 0 | 145 | 34,896 | MgCl2 | |
Y9 | 2600 | LS3 | 6.4 | 11000 | 567 | 1144.35 | 0 | 169 | 34,705 | MgCl2 | |
S-1Sa | 2301.8 | SY1 | 8.10 | 17179 | 388 | 512 | 12 | 1712 | 48,020 | Na2SO4 | 0.9 |
S-1Sa | 2311.6 | SY1 | 8.14 | 17603 | 365 | 452 | 15 | 1481 | 48,689 | Na2SO4 |
[
Since the strong randomness of calcareous interbed distribution, it’s affected by the combination of multiple factors, mainly divided into macro and micro perspective.
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 [
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 [
According to the stratigraphic thickness figure and drilling analysis, calcareous interbeds mostly develop in the paleo-tectonic highs (
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 Ca2+ with CO 3 2 − (calcium debris sediments transformed by fresh water) during the shallow burial stage, usually filling in the primary intergranular pores. While under deep burial conditions, the maturation of organic matter releases a large amount of organic acids, which facilitates the precipitation of calcium carbonate again by increasing the dissolution of carbonate and pH of the fluid, then Type III comes into being. It’s mainly found in interparticle or intragranular dissolved pores or replacing
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.
Zhu, P.Y., You, L., Yuan, Q.T., Zhong, J. and Liu, A.Q. (2018) Mechanism and Distribution of Calcareous Interbeds in Songtao Uplift and Its Periphery of Qiongdongnan Basin. Open Journal of Marine Science, 8, 370-385. https://doi.org/10.4236/ojms.2018.83020