water Article Determination of Paleocurrent Directions Based on Well Logging Technology Aiming at the Lower Third Member of the Shahejie Formation in the Chezhen Depression and Its Implications Yangjun Gao 1,2, Furong Li 2,3, Shilong Shi 2 and Ye Chen 1,4,* 1 School of Earth Sciences and Resources, China University of Geosciences, Beijing 100083, China; [email protected] 2 Shengli Oilfield Branch Company, SINOPEC, Dongying 257001, China; [email protected] (F.L.); [email protected] (S.S.) 3 Faculty of Land and Resources Engineering, Kunming University of Science and Technology, Kunming 650093, China 4 School of Water Resources and Environment, China University of Geosciences, Beijing 100083, China * Correspondence: [email protected] Abstract: The Bohai Bay basin, mainly formed in the Cenozoic, is an important storehouse of groundwater in the North China Plain. The sedimentary deposits transported by paleocurrents often provided favorable conditions for the enrichment of modern liquid reservoirs. However, due to limited seismic and well logging data, studies focused on the macroscopic directions of paleocurrents L are scarce. In this study, we obtained a series of well logging data for the sedimentary layers of Es3 Formation in the Chezhen depression. The results indicate the sources of paleocurrents from the northeast, northwest, and west to a center of subsidence in the northern Chezhen depression at that Citation: Gao, Y.; Li, F.; Shi, S.; time. Based on the well testing data, the physical properties of the layers from Es L Formation in Chen, Y. Determination of 3 Paleocurrent Directions Based on this region were generally poor, but two abnormal overpressure zones were found at 3700–3800 m Well Logging Technology Aiming at and 4100–4300 m deep intervals, suggesting potential high-quality underground liquid reservoirs. the Lower Third Member of the By combining with other geological evidence, we suggest that the Pacific Plate was retreating and Shahejie Formation in the Chezhen changing its direction from NE–SE to W–E and the Bohai–Luxi block was suffering an extrusion from Depression and Its Implications. NE induced by the Lan–Liao and Tan–Lu strike-slip faults in the early Paleogene. Water 2021, 13, 408. https://doi.org/ 10.3390/w13040408 Keywords: well logging; paleocurrent; provenance; Bohai Bay basin; Pacific Plate; Tan–Lu strike-slip fault Academic Editor: Constantinos V. Chrysikopoulos Received: 16 December 2020 Accepted: 1 February 2021 1. Introduction Published: 4 February 2021 The North China Plain has one of the largest groundwater stores in the world. The Publisher’s Note: MDPI stays neutral groundwater reservoirs are widely distributed in the sedimentary interlayers of the post- with regard to jurisdictional claims in Cenozoic age in the Bohai Bay basin (Figure1a), laying in the north of the North China Plain. published maps and institutional affil- The Chezhen depression (Figure1b) is a representative secondary sub-basin tectonic unit in iations. the Bohai Bay basin [1–3]. The study of this depression is of great help to understand the distribution of groundwater reservoirs and the tectonic genesis of the Bohai Bay basin. In previously published papers, many studies have focused on the stratigraphy, sedimentology, microtectonics, oil–gas exploration, and thermal evolution history of the eastern Bohai Bay basin [4–9]. However, due to limited seismic and well logging data, studies focused on the Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. macroscopic direction of paleocurrents are scarce, and the coupling relationship between This article is an open access article paleocurrents and the tectonic background has not received enough attention [10,11]. distributed under the terms and The migration of paleocurrents controls the sedimentary filling process and the compo- conditions of the Creative Commons sition of sediments in the basin, and it further influences the scale and physical properties Attribution (CC BY) license (https:// of underground liquid reservoirs [12–14]. In particular, the paleowater flow was always creativecommons.org/licenses/by/ accompanied by mineral transportation and sedimentary screening, leading to the depo- 4.0/). sition of mature sediments in the downstream area to benefit the modern preservation Water 2021, 13, 408. https://doi.org/10.3390/w13040408 https://www.mdpi.com/journal/water Water 2021, 13, 408 2 of 18 of groundwater. Therefore, the direction of the provenance of sedimentary deposits is a reliable indicator of the macroscopic direction of paleocurrents [15–17]. Furthermore, the state of paleocurrents reflects the paleogeomorphologic characteristics. In general, paleocurrents always flow from higher to lower altitudes, so the paleogeomorphologic features can be restored by using the results of paleocurrents to illustrate the dynamic changes of the surrounding tectonics [18–20]. Traditional methods for provenance analysis mainly include heavy mineral analysis, clastic rock analysis, sedimentary analysis, and fission track methods [21–26]. However, the experimental results of traditional methods are easily limited by the geographical locations of the sampling sites, the number of samples, and the erosion and chemical weathering of outcrops, leading to inconclusive results. In well logging technology, data such as gamma- ray and photoelectric absorption data are the sum of the specific information obtained from the target formations buried underground. Therefore, the required data for provenance analysis can be obtained directly from the original rocks [27,28], which greatly improves the accuracy. In recent years, well logging methods have been used to determine the directions of paleocurrents and the sedimentary provenance by quantitative and semi-quantitative calculation, and good results have been achieved [29,30]. In this paper, we obtained the seismic and well logging data from seven wells in the Chezhen depression focusing on the sandstone layers in the lower part of the third member L of the Shahejie Formation (Es3 ) in the early Paleogene. The directions of paleocurrents and the sedimentary provenance in this area were determined, and then favorable locations of underground liquid reservoirs were predicted. By combining the results with the tectonic background of the Bohai Bay basin in the early Paleogene, we interpreted the geotectonic dynamic of the Chezhen depression and the block it belongs to. Figure 1. (a) A simplified tectonic map of the Bohai Bay basin, modified from Li et al. (2011) [31]. (b) A simplified geological map of the Chezhen depression, modified from Lao et al. (2011) [3]. It should be noted that the logging data from the seven wells (yellow points) were used in most results; however, the reconstruction of the paleogeomorphologic map was performed based on all 40 wells in this region. The locations of the other 33 wells are hidden in this map following privacy policies. Water 2021, 13, 408 3 of 18 2. Geological Information Located in the east of the North China block, the Bohai Bay basin is the central region where the Paleo-Asian Ocean, Tethys Ocean, and Pacific Ocean interacted [32–35]. It is a large-scale basin where the Mesozoic-Cenozoic rocks are superimposed on the pre- Paleozoic basement of the North China craton [36–39]. The tectonic evolution of the Bohai Bay basin can be divided into three scenes: the first syn-rift and post-rift scene in the Jurassic (201.3–145.0 Ma); the second syn-rift and post-rift scene in the Cretaceous (145.0–65.0 Ma); and the third syn-rift and post-rift scene from the Paleogene (~65.0 Ma) to the present [40]. Our research was carried out in the Chezhen depression, laying at the southeast of the Bohai Bay basin (Figure1b) [ 1]. It is one portion of a series of tectonic combinations of uplifts and depressions arranged in a northwest direction; it is adjacent to the Bohai bay and is bounded by the Tan–Lu strike-slip fault in the east and the Lan–Liao strike- slip fault in the west [41–43]. The Chezhen depression is a Cenozoic continental faulted basin developed based on the long-term reconstruction of the cratonic basement [1,3]. It is a synsedimentary half-graben controlled by the Chengnan normal fault that connects with the Chengning uplift and a partially exposed Precambrian crystalline basement to the north, and the Cenozoic sedimentary overlapped cover extends to the Yihezhuang uplift to the south [44]. This pattern of W–E trending fault in the north and stratigraphic overlapping in the south may suggest a crustal stretching. The Chezhen depression can be further divided into three sub-tectonic regions: the steep south-dipping slope formed by the fault in the north, the gentle slope belt extending to the south, and the depression area formed inside [1]. The interior of the Chezhen depression has stable sedimentation and weak faulting, and only four weak secondary shear faults have developed in it. Thus, the Chezhen basin is suitable for drilling and well logging research in the depression. The Chezhen depression contains the ancient basement in the lower part and the strata of sedimentary cover generated during the three rift cycles in the upper part [1,3]. The basement is composed of a Precambrian metamorphic basement and metamorphic or unmetamorphosed Paleozoic sedimentary rocks, including Cambrian–Ordovician marine carbonates and Carboniferous–Permian clastic sediments and limestones. The cover consists of Jurassic to Cretaceous volcanic rocks and terrestrial sediments, as well as lacustrine and fluvial sediments in the Cenozoic (Figure2). The thick Paleogene strata are mainly composed of thick Shahejie Formation (Es) sediments, which are further divided into four members. Es4 is composed of a 200–1000 m thick red bed, mudstones with interbedded gypsum, and salt rocks. Es3 consists of 300 m to 1200 m gray-black sandstones and conglomerates. Es2 includes 100–600 m variegated mudstones and inter-layered siltstones. Es1 contains 50 m to 400 m sandstones, mudstones, shales, and biogenic limestones [3,45,46].