A computer-implemented method of determining subsurface target locations for horizontal wells to be drilled within a predetermined boundary, which comprises: selecting a reference well within the predetermined boundary; determining a first point and an azimuth direction at total depth of the reference well; creating a first line that is perpendicular to the azimuth direction and passes through a center location of the predetermined boundary; creating a second point on the first line at a maximum distance from the center location within the predetermined boundary; determining an offset distance, based on a predetermined well spacing distance and an initial offset distance along the first line between the second point and a third point defined by an intersection of the first line and a second line running along the azimuth direction from the first point, by subtracting the predetermined well spacing distance from the offset distance until the offset distance is less than the predetermined well spacing distance; creating a series of points along the first line beginning at a point which is offset by the offset distance from the second point on the first line until reaching twice the maximum distance, each point in the series of points being separated from another point in the series of points by the predetermined well spacing distance; and computing a list of heel/toe pairs for each point in the series of points and adding the list for each point in the series of points to a collection of lists comprising heel/toe pairs as each list is computed, the collection of lists representing the target locations within the predetermined boundary, each heel and toe pair representing a horizontal lateral section of a horizontal well to be drilled, wherein computing a list of heel/toe pairs for a selected point in the series of points comprises: creating a third line through the selected point along the azimuth direction; extracting points at which the third line intersects the predetermined boundary; and dividing the interval between the points at which the third line intersects the predetermined boundary into a set of equal length heel/toe pairs according to a spacing rule. selecting a reference well within the predetermined boundary; determining a first point and an azimuth direction at total depth of the reference well; creating a first line that is perpendicular to the azimuth direction and passes through a center location of the predetermined boundary; creating a second point on the first line at a maximum distance from the center location within the predetermined boundary; determining an offset distance, based on a predetermined well spacing distance and an initial offset distance along the first line between the second point and a third point defined by an intersection of the first line and a second line running along the azimuth direction from the first point, by subtracting the predetermined well spacing distance from the offset distance until the offset distance is less than the predetermined well spacing distance; creating a series of points along the first line beginning at a point which is offset by the offset distance from the second point on the first line until reaching twice the maximum distance, each point in the series of points being separated from another point in the series of points by the predetermined well spacing distance; and computing a list of heel/toe pairs for each point in the series of points and adding the list for each point in the series of points to a collection of lists comprising heel/toe pairs as each list is computed, the collection of lists representing the target locations within the predetermined boundary, each heel and toe pair representing a horizontal lateral section of a horizontal well to be drilled, wherein computing a list of heel/toe pairs for a selected point in the series of points comprises: creating a third line through the selected point along the azimuth direction; extracting points at which the third line intersects the predetermined boundary; and dividing the interval between the points at which the third line intersects the predetermined boundary into a set of equal length heel/toe pairs according to a spacing rule. creating a third line through the selected point along the azimuth direction; extracting points at which the third line intersects the predetermined boundary; and dividing the interval between the points at which the third line intersects the predetermined boundary into a set of equal length heel/toe pairs according to a spacing rule.