While drilling the Horse Hill 1 well, several significant hydrocarbon occurrences were recorded in limestones within the Kimmeridge Clay Formation, Oxford Clay Formation and the Lower Lias limestone-shale unit.  Hydrocarbons were also documented in additional units, but these are less significant than the aforementioned units.

The licence is underlain by a seismically defined tilted horst*, similar to the Palmers Wood oil field located 20 km to the north-east.  A well drilled by ESSO in 1969, Collendean Farm-1 (“CF-1”), in the north-eastern edge of the Horse Hill Wood prospect, found good oil shows at various Jurassic stratigraphic levels, which confirmed a reservoir presence with porosity of up to 27% and a net to gross ratio of 0.95, with an expected oil recovery rate in the region of 30%.  Recent seismic interpretation showed that the CF-1 well was drilled off-structure on the downthrown side of the bounding fault and outside closure.  The Horse Hill 1 well was drilled to the south of the fault, into the horst, and encountered hydrocarbons.

*Horst- A relatively high-standing area formed by the movement of normal faults that dip away from each other. Horsts occur between low-standing fault blocks called graben. Horsts can form in areas of rifting or extension, where normal faults are the most abundant variety of fault. 

Geology of the Mesozoic Weald Basinof southeast England. Oil fields (green), gas fields (red). Horse Hill licences (red polygons)

Stratigraphic column of the geology in the Weald Basin. Columns on the right document the known oil (green) and gas (red) field, and the presence of hydrocarbons in the Horse Hill-well. Lower case are shows, while capitals are significant occurrences.

Source: The Jurassic Shales of the Weald Basin: Geology and Shale Oil and Shale Gas Resource Estimation.

British Geological Survey and Department of Energy and Climate Change Report DECC 2014