4.1 Sequence boundaries and system tracts
4.1.1 Sequence boundaries
Two sequence boundaries have been identified-one is between the LCM and the Mudstone Member (marked “te1” in the seismic profile) (Fig. 3), and the other is between the Mudstone Member and the UCM (marked “te2” in the seismic profile) (Fig. 4). These three members constitute a third-order sequence where LCM corresponds to Sequence I (LCM) and UCM corresponds to Sequence III (Fig. 3).
(1) Identification of lithology
Unconformity, sedimentary hiatus, exposure, and stream channel incision are classic lithology identification marks (Powell 1875; Cross 1988; Adabi et al. 2016). Three completely different depositional systems develop in the Meihe Formation and their lithological association changed drastically from bottom to top: 1) the Conglomerate Member and LCM primarily contain fan delta sediments and thick coal seams deposited in lake swamps; 2) the Mudstone Member consists predominantly of semi-deep to deep lacustrine sediments of oil shale, siltstone and mudstone; 3) the UCM compose mainly of fan delta front and shallow lacustrine sediments. The thick coal seams in the LCM are overlain directly by the thick dark mudstones of the Mudstone Member, and this indicates a hiatus in between (Fig. 3; Fig. 5).
(2) Identification of well-log curves
The two sequence boundaries of Meihe Basin can also be easily recognized from the sudden changes of well log curves (Van et al. 1990), where up against the te1 and te2 boundaries, formation resistivity (Rt) increases drastically whilst gamma-ray (Gr) drops distinctively (Fig.3).
(3) Identification of seismic profiles
In Meihe Formation, seismic reflections usually emerge and terminate in the geometry of onlap, toplap or truncation. They either truncate under or onlap over the sequence boundary (Wang 2012; Yao 2012). Onlapping can be observed in both boundaries (Fig.4) and sequence boundary te2 is characterized by medium to high amplitude, intermediate frequency and continuous to discontinuous reflections.
4.1.2 Identification of system tract
The system tracts are the sedimentary systems that formed during the same geological time. A third-order Sequence is classified as a combination of the lowstand systems tract (LST), the transgression systems tract (TST) and the high stand systems tract (HST) (Olsen 1990; Shanley 1994; Vail 1991). The initial and the maximum flooding surfaces are the key differentiators for the classification as they always cause significant changes in the well logs and lithology. In well MH2011-10, two obvious and drastic changes in the logging curves correspond to these two surfaces. Above the maximum flooding surface are low SP, high gamma ray and high resistivity (Fig. 3). Coarse sandstone of the fan delta plain sediments is overlain by dark silty mudstone of fan delta front sediments at the first flooding surface. While at the maximum flooding surface, sandstone of fan delta front is overlain directly by coal seams of lake swamp (Sequence I) or coal or charcoal of fan delta plain marsh (Sequence III). Thereafter, LST, TST and HST are distinguished in the third-order sequence by using the two surfaces as markers.
Besides, the rare earth elements (REEs) can also be used to identify the system tracts (Tian e t al. 2006), where the total REE content (∑REE) increases from LST to TST till it reaches maximum at the maximum flooding surface and then decreases in the HST (Fig. 3).
In summary, both Sequence I (LCM) and Sequence III (UCM) contain LST, TST and HST. Due to the occurrence of a parallel unconformity between Sequence I and Sequence II, only the TST and HST are developed in Sequence II (Fig. 3).
4.2 Sedimentary facies variation
In this paper the paleogeography and lithofacies of each system tract in the third-order sequence were reconstructed using the single-factor analysis and the multifactor comprehensive mapping method proposed by Feng (2004). By integrating the study results of cores and environmental facies obtained from 134 wells and the stratigraphic correlations (Fig. 6) conducted for the key exploration lines, contour maps of various single-factors, i.e., strata thickness, sandstone thickness, sand content, thickness of coal seams and number of coal seams, were derived.
4.2.1 Strata and sandstone distribution characteristics
(1) Sequence I (the LCM, shown in Fig. 7-1)
LST (Fig. 7-1a): ① The strata thickness contour map shows that the strata are thickest in the areas marked as A, B and C, typically in a fan or leaf shape. The thinnest strata are mainly located in the northwest in the vicinity of boundary fault F1. ② The sandstone thickness contour map shows that the thickest sandstones are developed in area A, while the thinnest ones are distributed predominantly in the west. ③ The sand content contour map shows that the strata of the highest sand content are mainly located in area A and B, while those of the lowest sand content are near the western boundaries of the study area. From the above it is clear that area A has the highest strata thickness, sandstone thickness and sand content, indicating that direction A’ is the major sedimentary provenance during the deposition of LST in Sequence I.
TST (Fig. 7-1b): ① The thickest strata are in the areas of B, C and D, while the thinnest are distributed in the west wedging towards south. ② The sandstones are thickest in area B and thinnest in the vicinity of boundary fault F2. ③ The highest sand contents are found in area C at the north corner and the entire southwest including area B, while the lowest are distributed along boundary fault F2. It can be inferred from these three maps above that there might have been one major sedimentary provenance of B’ and one secondary provenance of C’.
HST (Fig. 7-1c): ① The thickest strata are present in the areas of D, E and F, in a fan-shaped distribution, and the thinnest are clearly in the northwest. ② The thickest sandstones are also located in the D, E and F, while the thinnest are vastly distributed in the middle west. ③ Sand contents are found highest again in D, E and F, and lowest in the middle west close to the boundary fault F1. These three maps reveal that there are likely to be two sedimentary provenances for the HST of Sequence I: the primary D’ and secondary E’.
(2) Sequence III (the UCM, shown in Fig. 7-2)
LST (Fig. 7-2d): ① There are four areas of highest strata thickness, three on the western boundary including area G and one near the eastern boundary fault F2, while the strata of the lowest thickness are distributed in three spots, the southeast, the northeast and near boundary fault F1. ② The sandstone is thickest in area G and a small central area, and thinnest predominantly in the southeast. ③ High sand content areas include part in the southeast and the majority of the north including G. Therefore it can be concluded that, during the deposition of the LST in Sequence III, G’ was deemed the major sedimentary provenance.
TST (Fig. 7-2e): ① Both the strata thickness and the sandstone thickness contour maps show similar distribution patterns, where the highest values are observed in the areas of H and I, and the lowest are along the eastern boundary. ② Highest sand contents are also found in H and I besides an area next to boundary fault F2, while the lowest is in a small area of the middle west. So we assumed two major sedimentary provenances, H’ and I’, existed during the deposition of TST in Sequence III.
HST (Fig. 7-2f): ① High strata thickness is found two places next to boundary fault F2, the one in the middle is distributed in a fan shape. Lowest values are observed in two places near the boundary fault F1. ② The distribution of sandstone thickness follows a very similar pattern. ③ Two areas of high sand content exist, one close to the middle west and the other in the vast north. The lowest sand content is located in the southeast. This suggests that sedimentary provenance J’ existed during the deposition of HST in Sequence III.
4.2.2 Distribution of coal seams
The Meihe Basin is a fault basin with widely distributed coal seams. In this particular basin, coal seams of 0.5 m thickness and above are considered economically minable. In this section we put forth our study results of both minable and unminable coal seams in relation with their cumulative thickness, layering, vertical distribution (Fig. 8) and planar distribution (Fig. 9) in various system tracts of different sequences (Table 2). Furthermore, we studied the relationship between the distribution of coal seams and sedimentary environments.
Table 2 Statistical results of layers and thickness of coal seams in LST, TST and HST of sequence I and sequence III
Coal properties
|
Thickness/layer
|
Sequence I
|
Sequence III
|
LST
|
TST
|
HST
|
LST
|
TST
|
HST
|
Commercial coal seams (>0.5m)
|
Total thickness/
single well (m)
|
/
|
0.52-41.54
|
0.84-45.19
|
1.67-1.93
|
0.5-15.06
|
0.61-14.83
|
Layers/single well
|
/
|
1-21
|
1-14
|
1-2
|
1-8
|
1-12
|
Thickness/single layer (m)
|
/
|
0.5-6.98
|
0.5-25.08
|
0.73-1.93
|
0.5-7.58
|
0.52-5.71
|
Average thickness/
single layer (m)
|
/
|
1.45
|
2.46
|
1.2
|
1.24
|
1.31
|
Unminable coal seams (<0.5m)
|
Total thickness/
single well (m)
|
0.08-3.14
|
0.04-7.55
|
0.15-4.88
|
0.17-2.27
|
0.13-2.98
|
0.23-2.42
|
Layers/single well
|
1-14
|
1-34
|
1-28
|
1-13
|
1-10
|
1-8
|
Thickness/single layer (m)
|
0.06-0.49
|
0.04-0.49
|
0.03-0.49
|
0.04-0.48
|
0.04-0.49
|
0.07-0.48
|
Average thickness/
single layer (m)
|
0.19
|
0.22
|
0.21
|
0.25
|
0.27
|
0.27
|
(1) Sequence I (The LCM)
LST: The coal seams developed in the LST of Sequence I are unminable. In the vertical profile, a few discontinuous thin coal seams are developed of fan delta plain marsh and distributed more widely in the southeast than the northwest (Fig. 8). Statistics from 134 wells show that unminable coal seams have a wide distribution with a total thickness of 0.08 m-3.14 m and up to 14 layers (Table 2). The thickness of a single coal seam is between 0.06 m and 0.49 m with a median of 0.19 m (Table 2). The thickness and number of layers both decrease from the northeast (close to the boundary fault F2) to the northwest (close to boundary fault F1). By synthesizing the contour maps of strata thickness, sandstone thickness and sand content, a palaeogeography and lithofacies map was produced. As shown on the map, the fan delta facies consist primarily of fan delta plain, fan delta front and shallow lake subfacies that developed from south to north. The shallow lacustrine sediments are mainly distributed in areas close to boundary fault F1 (Fig. 9-1a). By overlapping the contour maps of coal thickness and layers with others, it can be concluded that the discovered coal seams are mainly developed in the fan delta plain marsh of medium strata thickness and low sand content (Fig. 7-1a). Coals accumulated in the fan delta plain are easily affected by sediment supply and channel migration, therefore many layers of unstable thin coal seams formed, and cost wise, they are unminable.
TST:The coal seams of the TST in Sequence I have a wider distribution compared to the LST. They appear at the bottom as discontinuous and thin beds but gradually get thicker and more continuous near the top (Fig. 8). Statistics show that the minable coal seams are mainly distributed in the northwest, southeast and northeastern (Fig. 9-1b). The coal seams have up to 34 layers with a total thickness of 0.04 m-7.55 m, while the thickness of a single coal seam is between 0.04 m and 0.49 m with a median value of 0.22 m (table 2). Coal seams are mainly distributed in the southeast, thinning out to the north. The sediments of the fan delta plain and fan delta front facies were mainly deposited in the northeast and west. Compared to the LST, the fan delta of TST features a narrower distribution. From map overlapping, it is known that the coal seams in TST were mainly developed in the lake swamp facies of large strata thickness and low sand content (Fig. 7-1b). Distal to the sediment source, the lake swamp environment is hardly influenced by waves or channel migration and thus appears in large thicknesses and stable distribution. Among all the discovered minable coal seams of the Meihe Basin, those from the TST in Sequence I were formed the earliest.
HST:Coal seams in HST tend to migrate to the northwest. Minable coal seams are mainly accumulated in the northwest and northeast (Fig. 9-2c). Statistics show that the coal seams have up to 14 layers with a total thickness up to 45.19 m. The thickness of a single coal seam is between 0.5 m and 25.08 m with a median value of 2.46 m. The unminable coal seams are mainly situated in the southwest of up to 28 layers, 0.15 m-4.88 m total thickness and 0.03 m-0.49 m single layer thickness (with a median of 0.21 m) (table 2). The fan delta sediments were mostly deposited close to boundary fault F2. Lake swamp is the main depositional environment for coal seams that are distributed predominantly in the north close to boundary fault F1 ( Fig. 9-2c ). Due to the distance, the fan delta sand body exerted little impact on the in-situ accumulation of coals. Therefore these coal seams are characterized with large thickness and predictable distribution and hence greater economic value. In the vertical section, coal seams are thicker than in the TST, especially in the upper section. At the top of HST, the coal seams are the thickest and most continuous. During its deposition, the 12th coal seam, which has the highest commercial value, was formed (Fig. 8).
In summary, during the deposition of Sequence I, fan delta plain marsh and lake swamp are the main depositional environments for coals. The fan delta plain marsh is mainly distributed in the LST while the lake swamp is found in both TST and HST. In the sedimentary sequence of LST-TST-HST, the shallow lake facies are narrow and coal seams become more predominant. The coal-accumulating environments shifted from fan delta plain marsh to lake swamp facies. The TST is the earliest formation that contains minable coal seams, and more importantly, the HST bears coal seams of the highest commercial value in the study area.
(2) Sequence III (the UCM)
LST: Minable coal seams in the LST of Sequence III are distributed in limited areas of the north. They have a total thickness of 1.67 m-1.93 m and consist of only one or two layers with 0.73 m-1.93 m thickness each (median 1.2 m) (Table 2). Unminabe coal seams are mainly distributed in the northwest of up to 13 layers, 0.17 m-2.27 m total thickness, and 0.04 m-0.48 m (median0.25 m) single layer thickness. Fan delta was the main depositional environment and their sediments cover more than half of the study area. Besides, shallow lacustrine sediments are mainly distributed in the southwest. It can be deduced from map overlapping that the fan delta plain marsh is the main coal deposition environment (Table 1), but these coal seams are considered unminable with features of thin beds, discontinuous distribution and large numbers of layers.
TST: Compared to LST, coal seams deposited in the fan delta plain marsh of TST are more widely distributed. Generally speaking, however, these coal seams are thin and discontinuous. Across the entire vertical section, relatively continuous single coal seams are developed (Fig. 8). The minable coal seams of LST are mainly in the central part of the study area (Fig. 9-3e) as they have up to 8 layers, 0.5 m-15.06 m total thickness, and 0.5 m-7.58 m (median 1.24 m) single layer thickness. The unminable coal seams have a wider distribution (Fig. 9-3e). The unminable coal seams have up to 10 layers, 0.13 m-2.98 m total thickness, and 0.04 m-0.49 m (median 0.27 m) single layer thickness (table 2). Shallow lacustrine sediments are widely distributed in the central part, and a small portion of swamp is distributed close to boundary fault F1. What’s more, the fan delta sediments are widely distributed in the east and west, and fan delta plain marsh mainly located in the east and northeast is the main sedimentary facies for coal accumulation (Fig. 9-3e).
HST: The coal seams are relatively thin and discontinuous, however in the bottom of the HST some relatively continuous thin coal seams are developed (Fig. 8). The coals are more narrowly distributed compared to the TST and are mainly limited to the central part of the study area (Fig. 9-3f). The minable coal seams have up to 12 layers with a total thickness range of 0.61 m-14.83 m and layer thickness of 0.52 m-5.71 m (median 1.31 m). The unminable coal seams have up to 8 layers with 0.23 m-2.42 m total thickness and 0.07 m-0.48 m (average 0.27 m) layer thickness (table 2). Fan delta sediments are widely distributed in southwest. According to map overlapping, the coal seams were mainly accumulated in the fan delta plain marsh which have a deposition center close to boundary fault F1 (Fig. 9-3f).
In a nutshell, compared to Sequence I, the coal seams developed in Sequence III are distributed in a narrower channel and with a smaller thickness. In addition, Sequence III fan delta sediments are more widely distributed but its shallow lacustrine sediments are the opposite. The coals were mainly formed in the fan delta plain marsh and therefore uneconomic for mining. Nevertheless, the TST and HST of Sequence III are considered the third and fourth accumulating stages of coals in the Meihe Basin.